If it had been on purpose, why would they slot it in Kecleon's slot? Zero reason for that. Head of marketing for the game in Japan tweeting question marks doesn't seem like a good way to market it either - surely they'd tease it with some more useful message, and on a far more public channel (like the official Pokemon GO accounts for worldwide and Japan...).
It just reeks of a mess. I believe they're going to pretend it's intended as the damage is done already, but again - if it was intended, why on earth have it take up Kecleon's slot. (And why have Kecleon transform to Ditto too when it briefly appeared? Suggests to me that effect is a failsafe when trying to catch something that shouldn't exist...)
Double posting because I might have an explanation for why it appeared in Kecleon's slot!! Well, it's a long shot, but apparently Kecleon was actually the first generation 3 Pokemon shown before the games actually came out! It first appeared in the anime in the Johto League Champions series in July 2001, which is a year before Ruby and Sapphire were released in Japan!
So maybe it's a fun throwback for them? Replacing the first gen 3 Pokemon with the first gen 8? I'm still confused on if the Let's Go exclusive Pokemon is supposed to be gen 7 or gen 8, though, haha.
Maybe give people a teaser to generate interest beforehand? Not saying it's a
good marketing strategy but it seems remotely plausible that they would use GO as a tool like this. GO isn't just a spinoff anymore, but it's being incorporated into "main" games, so maybe they want to push more interest in GO as well? Also, maybe this is why they chose to use a weird looking Pokemon instead of a fluffy one like Riolu/Zorua/starters (some of the first announced mons of their generation) because they maybe
wanted to generate doubt? Like, if they used a model for a more traditional Pokemon-looking Pokemon, then the gig would be up. But if they use one of the weirder ones, it generates speculation for a few days? I have no idea what the long-term effects of each strategy would be, just generating ideas maybe.