I think this is true sometimes but not always.By the way the game mechanics and lore changes have been done, it's pretty obvious they do the game mechanics first and then try to figure out how the lore fits after.
For instance, in Worlds & Monsters Stacy Longstreet, the then Art Director says this (p 15):
The Scramjet team [= world concept team] took all of the D&D monsters and attempted to create a world where all of these monsters could live. We reorganized some of them and assigned them new backstories to explain their locations, their alliances, and their enemies. In some cases, these revisions meant that physical traits would need to change. In other cases we found it necessary to redesign a monster completely, taking mechanics we liked from creatures we did not. As a result, some monsters in this edition [ie 4e] will look familiar, some will be new, and some will remind you of monsters that no longer exist but have exciting abilities borrowed from something else.
That's describing a process that's more nuanced than just "mechanics first, story second". But equally, it's not putting canon on any sort of pedestal! The story elements are being thought of in terms of their function in the play of a D&D game, not in terms of their place in a story that has already been authored and told.