The problem with physical drives is that they can (and do!) fail. The authors point is surely that the backup would need to be checked periodically, and drive failures dealt with.

Does magnetic media like this (especially spinning disk) suffer from bit-rot? What about the possibility of mechanical failure?

I'd never rely on mechanical disks as the one and only backup of any data critical to me - a two tier approach of mechanical for fast retrieval, and cloud/online backup seems to be the safest bet.

You absolutely need a maintenance regimen if you want to be able to reliably retrieve files 4 years later. If you just unplug the drive and stick it in your garage, sure it'll probably work, but I'd say there's at least a 5% chance of failure.