I use the terms official and unofficial lore to describe lore that is released by Bethesda and lore that is not. Official lore includes the pocket guides and art books that come with some editions of the games, but does not include lore written by developers that has not been released by Bethesda.

You may accept unofficial lore because it is notably written by the same developers that worked on the games. But when a developer contributes lore to the game, whatever expertise they had becomes irrelevant, since they cannot control what Bethesda does with the lore. If Bethesda changes the lore, those changes become canon. This does not mean that a developer's unofficial lore is wrong, it just means that it is not canon. Lore is intentionally contradictory and open to interpretation, so what lore is right and what lore is wrong is irrelevant.

Bethesda has never said that unofficial lore is not canon, but by that logic, I could say that someone's fanfiction, theories, or game mods are also canon because Bethesda has not said otherwise. This is why you cannot say that only the writings of developers is unofficial lore, because unofficial lore by definition is not limited to any number of sources. Bethesda's authority is important because it is the only objectional standard by which lore can be canonized.

As soon as you say that one piece of unofficial lore is canon, you reject Bethesda's authority in favor of your own; you have made your own list of what is lore. You are then free to keep or reject any lore that you like or dislike, even if it is official, because you cannot accept or reject Bethesda's authority whenever it suits you without being inconsistent in your logic.

Again, you are free to enjoy unofficial lore however you like, but do not call it canon.