don't get me wrong, I too find their "acceptable documents for proof of funds" a little ridiculous, but then again, those rules are on their ToS and you did agree to it when you registered an account on their gambling site and they expect you to follow it. it is a shitty situation that I think you can't really win.
Where those rules are mentioned in their ToS precisely ? Are you able to quote the exact passage please? Because AFAIK it's false, it's not written anywhere in their ToS and it shows the common argument uses against customers, "It's written in the ToS you've accepted" is just BS to scam people.