I've never heard of it before so it must be a new app. I think the idea is nice and can be helpful for those who can't afford to buy a hardwallet and have only a small amount to store. however I don't think is going to ideal solution because phones are heavily and no would carry two phones instead of just buying a small device like ledger nano and have a piece of mind.
There is another app the same as this wallet mention by OP they have two wallets called airgap vault and airgap wallet which is the online one and the offline one as cold Storage.
Based on my experience they are just like the same as a hardware wallet but the differences you don't need to connect your other mobile phone to your offline wallet compared to a hardware wallet that you need to connect it directly to PC.
I have two phones both of them are Samsung the new one and the old one(No wifi already because I destroy the mac with Z3X to make sure no wifi)
Until now I use them as a wallet for some coins because they currently support few altcoins.
So I think this is good and much safer than a phone with internet connection and good if you don't have extra budget to buy a hardware wallet.
Never heard any issue yet about airgap wallets this is much safer. I don't know about the wallet from the first-page post I never heard of them too.