Don't believe people who say they will not know. 10 years ago maybe. But today they will know. However being Thailand you might slip through. But again... don't count on it.
Wait everyone.... not a law yet and they are still figuring things out. Don't panic. Agents are on the case and will sort things out. It is what they do and know the rules. This is Thailand and things always seem to be announced and then a month later disappear... maybe to do the same thing over again with same results. Take the TM30 panic... the tax I'd requirement... the up and down of the DTV... etc.
Go back again at a different time different day and see a different person and do again. If that doesn't work find another branch. BKK bank has NYC location. So as a US with Non O and residency certificate etc... should not be a problem. US embassy/Consulate if asked... will laugh and probably say you don't need it.