ignore Martin, he's an expert in his own circumstances but he doesn't know what he doesn't know.
Basically, for you, you're very unlikely to be asked to show 20,000thb cash. But it's still a requirement an the IO CAN ask for it.
It's usually reserved for circumstances where the IO is looking to deny entry to someone. Maybe someone trying to gain entry on their fourth visa exempt and they've had previous COVID extensions and ed visas and so on. Having insufficient funds (
*****
CASH) Makes it very easy for the IO to deny entry to this individual.
For you I wouldn't worry about it. There are NO ATMs before immigration at any Thai international airports that I know of.
pre COVID, almost everyone using Sado border was being asked to show
*****
or 200thb tip left in their passport.
I've only used that border twice but was only asked to show cash on one occasion ( which I had on both occasions). I think things have changed on that border now.