Why should they?
Why not simply reset the setting to the point where sorcerers and warlocks are there right from the outset instead of trying to pound square pegs into round holes and explain stuff that just won't make anyone happy. Setting purists will complain about the retcon and everyone else will be annoyed by the waste of page count.
This is why I keep harping on the idea of "inspired by". This Dragonlance isn't a retcon or a rerelease of the original. It's a new take on the base idea - a setting where dragons play a major role in the everyday lives of the population. Where a massive war between said dragons dominates the setting. It's a setting where good and evil are having it out but instead of angels and demons, we have mortals fighting.
I mean, what kind of retcon could add sorcerers and warlocks to the setting and still make sense? Never minding spell casting bards who have healing spells. Oh, and the fact that almost every single class has spells? Do rangers have to take the Test of High Wizardry? After all, any caster that gets higher than 2nd level spells is supposed to take the test. Oh, and exactly how do we retcon barbarians being able to fly? On and on and on.
Of course they aren't going to explain this. That's a rabbit hole that you'll never find your way out of.