On their website it says it is self-custody, but I personally would not trust it. Coinbase very likely has access to the keys.

The most difficult way to address this question is to figure out whether Coinbase can be trusted - is the wallet closed source, is there anything in the fine print that says they can take your money, etc

The easy way is to download an open source app like Blue Wallet or Muun and move the funds over there. (If it's a lot of money then a hardware wallet is ideal. You can get a trezor for about $50.)