I've asked this question to the developers several times with no answer, and I believe this is a fairly important topic.

According to "Pocket Guide to the Empire, 1st Edition", the Aldmeri Dominion was formed in the year 2E830, yet according to "An Introduction to the Lore of the Elder Scrolls Online" (a posting made by Zenimax Online Studios), it is the newest faction formed out of the three in the game, making it's founding sometime between 2E572 (the founding of the Ebonheart Pact) and 2E583 (when the game begins). Here's a time line:

2E 560-603 - Knahaten Flu
2E 572 - Akaviri invaders in Morrowind are killed by Great Flood and Second invasion of Tamriel started, Ebonheart pact formed
2E 583 - Events of ESO begin
2E 828 - Birth of Tiber Septim
2E 830 - Founding of the Aldmeri Dominion (according to the pocket guide to the empire)

During the Knahaten Flu, the Altmer did provide assistance to the Khajiit, which could possibly explain an alliance before 2E 830. The developers may try explaining this discrepancy as an error in the book "Pocket Guide to the Empire, 1st Edition". I can't think of how else it could makes sense.

Am I missing something?

Looks like you spotting a bit of inconsistency there. Can't find anything about any pre-alliances and the like, so either it was a secret faction, or the writers of the first pocket guide did not want to give the Dominion too much history and quickly write them off, in favor of Tiber.

I noticed the same thing when this game was announced and the date was announced.

Basically they will have to say "The author of the book made a mistake. Historians write long after the events actually happened and they are operating off of incomplete information."