The wallet you are talking about is close source and there is no way to know what is exactly happening in the wallet.
As long as the source code isn't available, I assume that it's not secure.
they claimed its decentralised
I haven't ever used the wallet in question, but I am sure it's not decentralized, because you always have to connect to their centralized server for making transaction or checking your balance.
Note that being non-custodial is different from being decentralized.
They allow you to have access to your private keys. So, it's a non-custodial wallet, but that doesn't mean it's decentralized.