I think the "recent introduction" rule depends on which branch, and maybe even which member of staff you draw.
We very recently tried to open a joint account at a Kasikorn Branch in Bangkok, where we already had individual accounts.
The lass told us that it was a new requirement of the Bank of Thailand that I produced a letter from my Embassy, not a Residence Certifcate, this claim was confirmed by her manager, but my question asking what the Embassy had to confirm, met with blank stares from them both.
No point in arguing but we made a call to Kasikorn HQ who confirmed there was no such requirement, old or new, she didn't mention any requirement for a RC.
She she suggested that we return to the branch and ask again and if we got the same response we should call her and she would speak to the team member, she gave us her personal number.
My wife didn't want the clerk to lose face so she suggested we went to the main branch on Silom, where our account was opened without further ado.