If they are telling the truth, and they are (in fact) a non-custodial wallet, they cannot steal your funds IF you don't share your seed, master private key, or derived keys.
However, if they are lying (and they're a custodial wallet disguised as a non-custodial wallet) or if they included a backdoor in their software, they can potentially steal from you.

Since it's a closed source application, there is no way to know any of this for sure... IF you get this solved, empty out your abra wallet and switch to a decent one instead (i'd like to recommand either a ledger or trezor hardware wallet...).