1) George Lucas has said that Anakin brought balance to the force by "doing away with the Sith and getting rid of evil in the universe". If Snoke, who we can probably assume is skilled in the dark side of the Force, was around at the time of Palpatine's death, doesn't that mean that Anakin/Vader did not in fact bring balance to the Force, and was not the 'chosen one'?
2) Even if we were to answer that Snoke wasn't yet proficient in the Force at the time of the Emperor's death, how lame is the prophecy looking now - Anakin brought balance for maybe 20 years at most! As Lor San Tekka says in The Force Awakens: "Without the Jedi, there can be no balance in the Force" - clearly whatever balance Anakin achieved has quickly been lost (and even those few years between ROTJ and TFA weren't any better than those of the old Republic!). But from watching the prequels, you'd think the balance of the force would be a big deal, or at the very least last longer than a decade or two?!