I think the theory is right if you look at the fact that you need one album to make the other.
Era 1 - Boy, U2's debut. October, more drums. War, focused mostly on drums.
Era 2 - Fire, a sound completely different then War. More of the "sonic landscape". Tree, Fire but with more of an American background. Rattle and Hum, just about 100% old school American.
Era 3 - Achtung, as we know U2 dreaming it all up again with dance music and techno in mind, but still rock. Zooropa, developed from the ZooTV tour with more of a tech influence. Pop, dance music but still rock and roll.
Era 4 - ATYCLB, basic U2, a little more raw. Bomb, even more raw and rock n roll (for the most part)
I know this was a quick explanation. But I hope I somewhat got my point across. I think that's what makes U2's discography so great. You can't have one without the other.