Actually what is written on the Thai embassy page is clearly stating that the result must be issued within 72 hours, not the test sample itself (see BOLD below in your quote). Seemed strange to me too, as I too would have expected the test sample time is the important detail, so I called the Los Angeles (USA) embassy to confirm that specific point. They confirmed that the test could be earlier, and the result issuance has to be within 72 hours. In the US this is important because in the US the tests are soooo slow, that it is not always possible to get a test and result within 72 hours.
My US tests had 2 day/times - "Collected on" and "resulted on". I saw others that had also a third for the day/time the sample was received at the lab.
When I landed in Bangkok on 17th Sept, then at the airport the date the doctor repeated aloud from my form was the issue date. So presumably that is what he was looking for also.
Now, how will anyone else interpret it ? Who knows. Maybe others are interpreting it as test also as the logical choice, even if the language doesn't exactly say it that way. Or maybe they just let the US slide because they are slow with testing.
You may be reading it wrong. It clearly says the test and a separate fit to fly certificate both within 72 hours of departure. That has been the regulation since the beginning and has never changed: see below from the London Embassy Website:
3. Fit to Fly health certificate, issued within 72 hours before departure – this is a statement by a doctor that you are free from symptoms and fit to fly (must be separate from COVID test)
4. COVID-19 test result with a laboratory result indicating that COVID -19 is not detected (COVID test must be by RT-PCR method), issued within 72 hours before departure