He's lying, yo. He's too bad a gambler to be using real money.
I am starting to agree with you, and I'm getting sceptical of his bets just due to the sheer size of them after losing so much in succession, however, I've been watching him bet for a while and seen him win his fair share, especially last season.
People shouldn't have to screenshot their bets, but I'd like to see David do it, since they're so big. There's no reason for him not to. Everyone on here would do it if asked. If he doesn't then he's probably lying, but again, it's not obligatory.
iirc, he posted a screenshot once.