Maybe because this story is completely fake. Maybe the government need to report at least a few cases in order to keep the emergency decree going, they will need to warrant their decision to close the borders. Perfect opportunity for this is to claim a bunch of recently returned Thais were infected.
The government will look like they were wrong to close the borders if 100% of returning Thais were clear of infection, they wouldn’t want this, they would rather say “see, we told you so”.
I’m not a conspiracy theorist but living so long in Thailand and knowing how this government operate, this would not surprise me.