I wonder why they bother? Do they actually get a lot ppl who arrive in Thailand but then run out of money?? And what does 20k THB prove? Same as a return ticket - it proves nothing since there is no guarantee you'll actually board your flight.
that is a rule from the 60's before ATMs, debit, credit cards, digital anything existed. Yes it's still on the books as a rule, but there is no chance they will ask. Yet whenever this question is asked the sticklers insist on pointing it out LOL