Tempered glass is particularly vulnerable from sharp, hard objects. This is the idea behind "ninja rocks" : pieces of broken spark plug ceramic that are used by criminals to break side windows of cars.

I'm not saying that Tesla windshields are vulnerable from that, in fact, most windshields aren't. However it shows that resisting a blow from a hammer that is neither sharp nor hard is not a conclusive test.

Worth noting that while side and back windows are tempered, windshields are typically laminated.

The difference is in how they break. Laminated glass will generally crack, but otherwise stick together, while tempered glass will shatter, but into less-sharp pieces (to minimize the hazard of broken glass).

I don't know how much of this matters, but it stands to reason that what works on a side window might not be the same as what works on a windshield.

I've tried smashing a windscreen with a hammer. The first few blows didn't do anything at all. All windscreens are very strong.