Technically I guess...
The thing is there's no official statement as to what a generation is, even though it's been mentioned in some media.
I keep wondering, even if it's not THAT official, I've recently bought all these sticker books, or another one that has like every Pokémon shown in posters in scale, to show the size comparison, and they usually divide/classify Pokémon in regions/generations. One of the books included only until Blacephalon and the other one does include Zeraora. When these books keep coming in the following years they will have to include Meltan and Melmetal somewhere. It will be odd that they put them in Alola, but would be weird too if they put them in a whole section just for them.
Now, what would happen if Meltan is available in Galar? The debate will never end, because some people will keep saying it's from Gen VII because it came out in Let's Go / Go. But others will say it's from Gen VIII because it's from Galar region.
I had hope they wouldn't be given a Pokédex number until the Switch games, so they would turn out to be 900-something, but they were already assigned 808 and 809, putting them at the end of Gen VII, or the beginning of Gen VIII (like Victini in Gen V) depending of which team you choose lol