Cool. It will be interesting to see how they will implement their bitcoin transfers. Hot wallet? Cold storage? etc.
Some wallets, like Freewallet, have you send them the coins and then they keep them in cold storage for you (no personal possession of the keys).
It looks like the Cash app holds fiat balances for customers so they will probably hold BTC in storage for customers.
Yes, hopefully they have learned from all these other exchanges that have been hacked...