I wouldn't think it would be bad advertising, seeing how Gen 9 was hardly marketed enough like the previous Gen games. That speaks alot volume that the anime will no longer just focus attention one region and instead promote all the regions.wouldn't doing one of the most criticized aspects of Journeys again be bad for a show that's basically an advertisement?
I think both gamefreak and the anime team realize that fans want to be reminded of the other regions and the pokemon, which is the reason why many older gen pokemon have returned in previous games and in the new series. A perfect example, is how Ash in the journey series was given a team of old gen and new gen pokemon, which catapulted alot fans to watch the journey's arc after realizing older gen pokemon would return.
I still remember how the pokemon twitter went up when fans saw Ash catching a Dragonite. That feedback pretty much confirmed that re-visitng old gens was a very marketable move. The fact that Pokemon has been an ongoing franchise for so long with pratically over 1,000 monsters, makes all the most sense for the company to re-introduce older gen and while promoting newer ones in the anime by having Ash travel the world.
If they want to promote the new gen, all the anime has to do is introduce a new character and use them to promote the new generation like they did with Goh, while focusing on Ash revisitng the older and new pokemon via legendaries and mythical pokemon.