Game mechanics don't necessarily take up the most man-hours. A lot of it is number-crunching and playtesting; it's non negligible for sure, but it's not like an xpack adds nothing gameplay wise irrespective of what it might remove; dungeon and raid bosses don't appear out of thin air, nor do Azerite effects. Stuff like art assets, music, animations, cinematics, that's where the lion's share of the work is, and in that regard BfA is no worse than any expansion, and better than anything apart from Mists and Legion if you ask me.
Does it take up exactly as much man-hours as crafting a game like, for example, Sekiro? overall I doubt so, but I think it's roughly in the same ballpark in terms of art assets because that game also reuses said assets a whole lot, as does literally every single game in existence, plus WoW goes back and updates old assets as well which mos games do not do. Gameplay-wise obviously Sekiro took more work, but BfA was also less expensive. Then again it requires a sub, but then again Sekiro might get one or two DLC's worth of content with a few areas and bosses, a WoW expansion gets several new zones, tons of activities, vast amounts of new rewards and collectibles which all take time to make, doubles its raid boss count at the very least, new cutscenes and story quests, etc. bought and paid for by subs.
I do think the Blizzard guy is exaggerating, but it does prove a point. WoW expansions are no joke, a lot of labor is put in them, and doing their best to avoid crunch time while maintaining the steady stream of content an MMO demands must indeed be a challenge which they take on, to their credit I must say.