-2 DEX is still 1AC and a lesser chance to be hit, heavy armor or not.
....No.
Heavy armor prevents dex from affecting AC. If you have 20 Dex and heavy armor worth X AC, you still only have X AC, no dex boost.
Medium limits you to +2 and Light gives all.
The idea is that everyone gets roughly (but not exactly) the same AC after the dex heavy classes wear light armor, middleground classes wear medium armor, and dex=dump stat classes wear heavy.
