It is a mistake, yes. But this particular mistake would have behaved very differently on a normal server. Just because there exist mistakes you can make that would have the same consequences on regular server vs serverless doesn't mean you can just shrug your shoulders and say all mistakes are the same.

The fundamental issue here is serverless is great at allowing you to automatically scale to meet demand, but it also is great at automatically scaling to meet unexpected resource usage caused by errors (or poor design). And so this means a mistake on your end can cost you a lot of money, because the system thought that it was real demand.