They got the names from a source. Which the people they got it from who got it from the source claim to be legit.
Personally, you have to be stupid to say the list is fake. No, I am not calling you stupid.
Edit at 8:49 AM (this was posted at 7:50 PM my time): Ha, Serebii just updated with the full list of English names. The exact same list that PokeBeach did.