I'm not sure how usage stats are weighted, but I'd take this with a grain of salt - anybody who has played high ladder has definitely come across some guy named empoleonlove, who spams
xtrashine's HO team like all the time (like seriously, I've never seen a dude with a higher battle count, it's like in the thousands this dude probably plays 30 battles a day).
If you look hard enough, you can see how this impact the stats:
1) EVERY MON ON THE TEAM HAS THE OTHER FIVE MEMBERS LISTED AS ITS FIVE MOST COMMON TEAMMATES LOL
2) Dragonite and Azelf have higher usage than shit like Breloom, Kyurem-B, and even essential stall 'mons such as Quagsire... and we all know how common stall is on high ladder.
3) Excadrill's most common set is apparently Choice Scarf... it's more widely used than the next four common items put together.
It's thus not hard to imagine that CB is bound to be more common than Leftovers on Talonflame, specifically because of this one player/team. But yeah, I agree while it's certainly not the best set it's a bit much to call it completely irrelevant.
Also, regarding Mega Slowbro... idk, I find that shit so underwhelming. Like it seems so scary but then shit like Rotom-W can literally 1v1 it and keep it in revenge kill range by burning, then constantly volt switching/pain splitting like lol. Also, we live in a wallbreaker meta (although Mega Slowbro does counter Mcham nicely) so it's not likely to get a lot of setup opportunities.