How can we be sure that what's in those wallets is 10% of what it should be rather than 1%? AFAIK, none of the documents from Binance are public, especially regarding their finances.
Not sure I understand. WalletA has $300m BUSD and WalletB has $420m BTC. Both wallets are 'owned' by Binance. This is lower than the $1b funding on Jan 29th, 2022, but it is still a significant amount of "real" (if you believe crypto has value) funds. This is just an insurance fund that we are talking about here.
I think you're referring to the rest of Binance proof-of-reserves, which are here. Note, I fully realize this isn't perfect.
"Binance’s SAFU began in 2018 by allocating 10% of the trading fee into a fund that is solely dedicated to backing up user holdings in the case of an incident. In February of this year, the fund hit $1 billion for the first time."