Some things on Earth (especially in the ocean) you'd think were extraterrestrial... What a gift to still be able to find such amazing animals out there.
They all, so far, share the same basic biochemistry, derived from the Last Universal Common Ancestor (LUCA).
What would be extraordinarily interesting would be if we could find life on Earth with a fundamentally different biochemstry. Very different genetic code, even. This would be sign that Origin of Life is not the Great Filter. And we don't even have to go to another planet to conduct this search for "alien" life.
An apartment inside an apartment complex is still inside the same building. Earth is in the Universe. There's a difference between "in the Universe" and "outside of Earth".
A superset also includes everything in all its subsets.