Just noticed this comment when I was looking through my messages for an old comment, and I wanted to respond.

It is the word "too" that is important there, and the usage you describe is only used as an affirmative for contradicting a negative statement (at least, that's proper grammar anyway).

For example, if the original statement had been "God must not make a boulder he cannot lift!" and I had responded with "God must too make a boulder he cannot lift!" you would be right, but the original statement is an affirmative statement ("God can make a boulder he cannot lift."), my own sentence before it is an affirmative (in the grammatical sense - not so much in the "uplifting" sense), so trying to contradict either with an affirmative doesn't make any sense.

Also, I did a Google search, and while using "too" between must and another verb is not common, using "must too" to mean "must also" is by far the most common usage I could find. I do admit that other combinations of verb "too" verb seem to imply contradicting a negation even without the proper context, so that usage is definitely not as clear as I originally thought it would be. I still think it's pretty, though.