Like, I'm not an expert on this, but I agree with the henchman here. Honestly earning extraordinarily large amounts of money in Las Vegas should be pretty difficult. They are very loathe to part with it. Even with magic or some-such—if you start earning too much money, the casinos will ask you to leave. And they're probably fairly sensitive to all sorts of fishy business by now.
Well, I guess the sort of tens-of-millions figure they were talking about might be possible, in... theory. Especially if it's other people's money (like in poker), instead of the Casinos'. But my gut reaction is at least broadly the same as the henchmen's: If not for plot armor, whatever he's planning would probably get him banned from all the casinos before he got the money he needed out of it (and he probably wouldn't be welcome at a mob-run auction after that, either).
It's not like "expert on casino best-practices" is believably on this guy's character card... Well, we'll see.
Edit looking at very first panel of next chapter. Yeah no. That's... not how it would work. XD