I think this may be the intention. Doesn't make much sense for the U.S. to donate vaccine in this kind of situation with a stipulation that 20% has to go to its citizens, let alone the citizens of its allies. Most likely there were no stipulations and the Thai government is preparing its citizens to accept that in order to quell the outbreaks a significant portion should go to foreign factory workers. Or if the stipulation did come from the U.S. it was to help ensure that the donated vaccine would be as effective as possible in slowing down the spread and mutation of COVID.