It seems like this article is aiming towards shared servers where you actually allow shell login to "untrusted" users, which IMO is a relic of days long past, that only really persists at (maybe) universities. Hell, even at the university I last worked at 6 years ago we just gave everybody their own VM. And nowadays I wouldn't even need to give them that... students can download and run a vagrant environment on their personal machine in like 2 commands.
It's not to say there's no audience for articles like this... I'm sure there's plenty of environments out there that still follow the multi-user server model from the 1970's, and I certainly pity anyone who has to administer those types of systems. But it certainly should be no surprise to anyone that there's a lot of malicious things you can do if you have shell access to a system (or to your point, the ability to upload arbitrarily-named files with arbitrary content. shudder)
Now all you need is a user who kindly requests for a copy of uploaded files. If you're not aware of this issue (and you must be "actively aware" i.e. watch out for it all the time) you can do something that shouldn't be possible.