Yes, the nubs still can be broken if you turn them, but no, this cannot accidentally happen, you really would need to be WANTING to do that.
When they're inside the case, it's more like "Can the display be broken if you press too hard on it" which also is a yes.
You can spin the nubs about 25 - 30°, then they have a bit of a stop. You can't spin them further if you keep keep rotating your thumbs - your thumbs will start to slide. That's how much force you need to do so.
The only way to break them with spinning is when you hold them from the top with your thumb and index finger and force-rotate them.
This is how it happened at the GamesCom - however, that only works when the PCB is not inside the case, as you then have enough space to actually hold the disc between your two fingers to rotate it.
When the unit is assembled, you only have the top of the disc available, and when you try to grab it between your two fingers, the case gets in your way.
So it's not easily possible... maybe if you use crazy glue to attach your thumb to the nub disc first, then you could start rotating
BTW: I'm also a nub rotator while waiting, and I even do it on the Pyra. It's even more fun rotating it, as the springs rotate it back automatically