I think it's fine. I haven't played MH or whatever game they're comparing it to, but if it's something like Spyro where it has open levels / areas, and like a main base where you can access to each of them, that works for me.
What is more relevant I think is the evolution it shows from other Pokémon games. The gameplay, the camera, the openness of the battles, etc.
People are always comparing Pokémon to other franchises, but the comparison should be to older Pokémon games.
I don't know if we'll ever get a fully open world in Pokémon, nor I'm sure it would really work, but between Legends Arceus and the Wild Area / DLC's of SwSh, I think we can expect better things for Gen IX.
When I hear an open world Pokémon game, the first thing I think it's a game where you're a Pokémon, like Pikachu or maybe a few more other options, and you just go and explore the entire world, battling other Pokémon, exploring complex areas, like caves, waterfalls, etc Something like Mystery Dungeon, but open world lol
The formula of the main games just doesn't seem suitable to me for an open world, because it will always require this little bit of progression, where you have to level up your Pokémon, teach them moves, evolve them, etc, but also having the gyms or equivalents as these milestones you have to reach. It doesn't make sense to start in a town and then go to the farthest gym and be like, I want this to be my first badge, when your Pokémon will most likely be overpowered after crossing the entire region.
I don't know what similarities there will be between Legends and the traditional games, but I doubt it'll lack some degree of completion like I mentioned. Like it wouldn't make sense to allow us to go straight to Dialga and Palkia or Arceus, or any of the stronger Pokémon without first going for the Bidoof, Budew, Starly, etc. or whatever people want when they think of open world.