Flash drive is, first and foremost, not an alternative, because it ain't a wallet. Hardware wallets are not "flash drives" specified for Bitcoin. They do more than just storing files. They sign transactions, to be precise. They are considered safe, because you don't leave your private keys in plain sight to your Internet connected computer. Lastly, we shouldn't give him precautionary measures about flash drives, because it's a dumb manner to back up wallets in the first place.

This article must have been written by an amateur. You should not rely on a flash drive which has a very limited lifespan. You should back up seed phrase in paper, and in multiple places. That's enough.