Like someone above said, it's a self reported system.
You add the game you're playing and then select which platform you're playing it on, and which storefront you bought it from. Switch 1 and Switch 2 are separate in there, so I'm guessing Switch 2 games don't have enough players on the HLTB platform to register in this list yet.
It's why PS2 is still on there. People are still playing and recording their playtime on PS2, adding it to their profile/catalog/etc