On the topic of generation, I do want to say, while this certainly does not prove that Meltan and Melmetal are Gen VII Pokémon, it does seem to reinforce the idea. The easy counterargument to make is that this is a Victini situation, with a number 0 Pokémon in the Gen VIII's regional dex. The problem with that is we have two Pokémon. So, if they are Gen VIII, either both of them are numbered zero, one is ridiculously numbered -1, or they actually push the starters away from their traditional numbers. All of these options seem unlikely. Admittedly, that last possibility is kind of realistic, but probably shouldn't be expected.
Once again, not claiming this is proof that Meltan isn't Gen VIII, but it certainly seems to imply it.