This is horrible advice. Anything connected to the internet is never considered safe, especially your crypto.
There are many malwares on many smart phones, especially Android devices. There could be some virus, which could trace your keywords on your phone and when you unlock your mobile wallet, it can send the funds to the hackers address. Or it can install some type of clipboard virus or QR code virus which automatically changes the address that you send your coins too. Your phone is always communicating with the outside world, because its made to do that, however you have no idea what info it sends to which parties. Don't keep any crypto, especially large sums on a mobile device. Either use Electrum/Armoury in cold storage or just get a hardware wallet.