Tyranitar can make mountains crumble using earthquakes and Macgargo is hotter than the surface of the sun. It's dex logic.
I think the main difference here is that it isn't a Dex entry. I definitely subscribe to the idea that the Dex entries are written by the literal child player character who sometimes just makes shit up or heavily exaggerates. When it comes to Volcanion apparently leveling an entire mountain range, that doesn't come from the unreliable player character but rather the editor of a newspaper. It could still be bullshit though, like people unironically believing aliens carved the pyramids with space lasers or whatever. Maybe the editor is the Pokemon world equivalent of a QAnoner.
Also people (including past versions of me) overblow the Magcargo thing. The surface of the sun is 'only' ~5800K, which is about 5 times hotter than the hottest surface lava ever recorded on Earth. It's hot as fuck, it would kill you if you were too close to it, and it would slowly melt its way into the mantle of the Earth, but it wouldn't instantly vaporize everything within a 10-mile radius or any of the other ridiculous things people say. Being 'as hot as the surface of the sun' does not mean releasing as much energy as the surface of the sun (turns out most of the energy the sun produces doesn't come from the surface, who'd have guessed). People assume the physics work out the same way as if you magically teleported a Magcargo-sized chunk of the sun to the earth, but that's just not the case.
The real danger is if you send your Magcargo out in water, which could potentially cause a massive littoral explosion that would probably result in a very unpleasant death for anything nearby. Turns out that 4x weakness to Water goes both ways.