Dragon magazine (and its content) was "sanctioned" [officially/by the creators] and widely accepted. Therefore, Canon. Lots of Forgotten Realms lore first appeared in Dragon, and is/was canon, and only later became "official". Widely accepted is the key point.
You don't need official sanction, by the way, to be canonical. Often official sanction convinces people to accept something as canon, but it is not necessary. Case in point - Which is canon and which is official: Han Shot First? Or Han shot Second? Officially its the later, but the first is widely accepted among the fan base and can be considered Canonical. (note: schisms are often a contest of canon. Offically, G. Lucas can say what he wants about the IP, but the fan base has other ideas about the canon and so we have two canonical views in schism).
Luther would not have succeed if the dogma of papal indulgences were widely accepted in germany. They were not, so to many of the northern Catholics, it was not canon, despite its "offical" status. To many others it remained canonical (an offical). Thus a schism developed over the differing canon. The schism was resolved in forming a new "official" canon of the Protestant churches vs the "official" canon of the Catholic church.