I don't think they were "intended" for anything - it's just that was the state of the control lines after it decoded that instruction byte, and combination might do something somewhat sane.

Wiring all the "illegal" instructions to a NOP would have taken a fair bit of extra logic, and that would have been a noticeable chunk of the transistor budget at the time.

That's exactly right. There's a really good article about it here: https://www.pagetable.com/?p=39