You are misreading the piece: the canon policy is for the benefit of the movie producers and toy designers, not players. They expect media partners to stick to depictions in the Monster Manual and Player's Handbook according to the most recent errata, but they don't expect the movie producers to stick to every detail in the Sword Coast Adventurer's Guidde let alone 2E material.Yes, that is the choice we can make. It's the choice we could always make, and many have made (including me!).
But Wizards isn't encouraging you to pick and choose with their current policy. They encourage you to imagine the lore of each edition as limited to that edition, and for the 5E lore to only include 5E material (and not even all of it). You can make up your own canon, they say, but they don't encourage mixing. (The implication I get is that they'd rather you use 5E lore, or modifications thereof.)
If they really wanted to encourage players to tap into the lore of every edition, they would have said that instead, rather than creating barriers between different editions' canon. Or just not said anything at all.