Yeah, agreeing here, and I think those are some good examples of more "complex" metamorphosis patterns in the real world.

I also found his other argument why Pokémon "evolution" would not be metamorphosis not very convincing either: Essentially, that Pokémon in-game and in-universe are only ever referred to by their "forms" and not by their "species".

That's true, but might simply be a cultural thing: I think I read somewhere that it took quite some time in the real world before people realized that cicadas and locusts are not just the same species but even the same individuals. You could imagine the same in the Pokémon world: People might have encountered Bulbasaus and Venusaurs separately, and when someone found out later that one can morph into the other, the names had stuck already.

Or it might just be more practical to refer to the forms if the metamorphosis get more complicated than a linear chain: You could probably find a good name for the "Bulbasaur/Ivysaur/Venusaur" species, but how about the "Eevee/Flareon/Jolteon/Vaporeon/Espeon/Umbreon/Leafeon/Glaceon/Sylveon" species?

I'm kinda reminded of the Zerg: Almost all the Zerg units start from a larva and then go through one or several stages of metamorphosis to become their intended unit. Still, it's obvious it's all just a single species ("Zerg") and its metamorphosis pattern is essentially a huge tree. So everyone refers to the individual unit names for identification and not the species name.