of course I don't believe that specific troll post. But what I say is that it's not out of place for an IO to check you still fulfill the requirements of the visa you have. I mean, they've done it all the time with other types of visas too.
Participante anónimo then one can tell to the officer: "I did my muay thai course already, and finished it. Now I'm just a tourist (on the DTV) for the remaining 4 years, any problem with that?"
that's a gray area, then. It's technically a tourist visa, but one of the requirements to grant it is to have certain money in the bank for several months. I'd assume it should be more appropriate to show one's bank balance than cash, but who knows. I normally have more than 20K on my TrueMoney always, which is literally like cash for all Thai matters.