The ACH Direct Deposit and IDD Direct Deposit are two different systems.
The Automated Clearing House (ACH) is the primary funds transfer system used in the U.S. It's primarily a "intra-US/domestic" transfer system but it does have international transfer capability if a bank in a foreign country is enrolled in ACH. Like how Bangkok Bank here in Thailand has ACH "receiving" capability through its "New York branch" which is a licensed US bank. A key component of the ACH system is each bank is assigned a unique 9 digit ABA number....like Bangkok Bank NY branch ABA number is 026008691. Just like a social security number is unique to an individual the ABA number is unique to a bank. And if you have paper checks you can see your bank's ABA routing number printed at the bottom along with your account number.
ACH uses two different file/transfer formats....a domestic format with specific underlying coding and an international (IAT) format with specific underlying coding. The IAT format is really just a domestic format but with more detailed info. And that underlying coding "which is all important" like how a domestic ACH transfer will usually use a Standard Entry Class (SEC) code of PPD for most direct deposits...PPD stands for Prearranged Payment & Deposit. However, for an ACH transfer "leaving the US" it has to be in the ACH IAT format....more detailed info...and uses an SEC of "IAT" which stands for International ACH Transaction. This coding is all important as it allows banks/authorities to easily and quickly identify/evaluate transfers....all the computer systems need to do is say "show/report to me all transfers with SEC "IAT". And this makes it easy for an bank computer system to accept or reject any transfer....like with Bangkok Bank they will simply need to set their computer systems to only accept transfers were the underlying SEC is "IAT". No manual review required....easy for the computer to spot and reject.
The International Direct Deposit (IDD) system uses the SWIFT system and the key code/number used it the 8 to 11 digit SWIFT/BIC code....for Bangkok Bank that code is BKKBTHBK (8 digits)...or BKKBTHBKXXX (11 digits) is fine also....the last three digits are optional and just represent a specific branch that banks some might want to include in the SWIFT code. The XXX in this cases just means the head office. As you may know, a SWIFT transfer includes detailed info on the sender and receive like how the ACH IAT method also requires. No SEC used in the SWIFT system....a SEC is unique to the ACH system.
As for the part you quoted that was from Danthai's post. Where he talked "new" signup form as in the SF-1199A form the latest version Aug 12 but a Jun 87 version is still in use by many because the Aug 12 version is really just been updated to be more user friendly for completion on a computer compared the the Jun87 version that is meant to be filled in by hand. But both still require the same core info such as name, address, routing number, bank account number, etc.,...there is no difference in terms of how one would result in IAT format being used and the other non-IAT format.
In fact, when I went to HQ Bangkok Bank yesterday with my family member to have them sign-off on the new SSA IDD for Thailand form I asked them are they now using any "new" direct deposit sign-up forms....they said No....still only using the SF-1199A for ACH Direct Deposit because they still haven't received any implementation guidance from the Bangkok Bank New York or SSA....as far as they knew the IDD was still in the testing phase.
The rep had just became aware of the new IDD for Thailand form as we all just have....since it's a Feb 2019 form only published a few days ago by SSA. I say "HQ" Bangkok Bank because my branch is the branch on the first floor of the HQ Bangkok Bank building in Bangkok, and at this branch to get US Govt Direct Deposit setup they sent you to the second floor of the building which handles the signup, incoming transfers to Bangkok Bank branches as a whole across the country, sets policy for all Bangkok Bank branches in Thailand, etc. And the rep I talked to is the rep on the Bangkok Bank 1 Oct 18 letter regarding the IAT issue.
The rep also said a person with military retirement pay coming to Bangkok Bank had brought in a DFAS IDD form for completion which she showed me a blank of. I said, yea, seen that before....it's from the DFAS webpage...it's generic IDD signup form "SF-1199-I", but unfortunately Thailand is not on the DFAS approved IDD list which is shown at below link. I've contact DFAS several times over the last 6 months about "Do you do IDD to Thailand or is something in the works to add Thailand to the approved DFAS list?" The answer was always No....the list of approved countries is shown at below webpage.
But Bangkok Bank still signed off on the IDD form because when a bank signs off on that area of the form they are only confirming the correct bank info is being used like ABA routing number, SWIFT code, bank account number, bank address, etc. They will sign off on any type of Direct Deposit form as they are many different forms used by different govt agencies and civilian companies. SSA has its IDD form, DFAS has it's IDD form, VA has it's IDD form, etc....ain't no standardization among US govt agencies paying benefits in terms of forms used and which countries they will send to.
Actually, a person don't even need to use these type of forms to sign up for direct deposit "if the US govt agency you are dealing with" also allows other methods like updating your Direct Deposit info online, calling it in, etc. Unfortunately, regardless of the method used to signup/change Direct Deposit it may not result in your Direct Deposit being sent in IAT format as that will also depend on other factors such as the address you have on-file with a benefit paying agency, does the agency also use IAT format, and other factors.