I suspect it does and that an ac of -120 is the same as having +12 armor from old school ad-d. I e base ac is 10 and +'s are actually subtracted from it so ac would be -2.

Am I close?

The answer to your question is "sort of"; however, note that this only applies to hitting and missing and that dodge/parry/etc. is a whole other ball of wax.

As far as how the specific numbers work, I believe it's somewhat more complicated than that.

Heros always hit then. Parry and all that not included.