He is testing: testing that people want this. In the beginning your biggest risk is making something poeple don't want, not code that breaks.
The tests can be the same or even better that those added at some TDD style.
And, in either case, that would be a nice problem to have.
And yes, it is a better problem to have, then to have code that noone wants.
Notch is creating right now, watch http://vimeo.com/36579366 "inventing on principle" which explains the importance of immediate visual feedback.
Also give him just a "little" credit for minecraft
I just dont want anyone else to think it is a good idea for them :-)