Drilling through the generic zinc pot metal that everything that can't be made of plastic seems to be made of nowadays could leave one with a wildly inaccurate idea of how difficult it is to drill in hardened steel.

Sure there is. It's simple. If there are abrasives and cutting tools hard enough to drill rock, they will also carve through whatever steel alloy you care to bring to the party. Admittedly brick isn't the optimal choice for demonstrating my point but I can't find my favorite video of this guy drilling a sink drain out of a solid granite boulder so... Also, we are talking about ATM housings, not the internal cash box. You really think they're constructed out of unobtanium?

It's not fast but it's possible to cut steel too with cordless drill: https://www.youtube.com/watch?v=aj12WdVhOCc

Took him 8 minutes on what's probably mild steel. A high security enclosure might use something like hardened cast vanadium steel with carbide chips. You'll go through a sack of those drill bits even going through half an inch.