I hadn't seen a replay yet, but yes, he was clearly safe. Unfortunately, the ump doesn't have the benefit of that still pic to make his call with, and as has been argued on these boards before, players sliding head first at firstbase can sometimes mess with the ump who's used to looking for a foot come down on the bag. When a foot comes down on the bag, the ump can see it clearly - when a hand is sliding in from the front side, the ump has the bag partially blocking his view. Is it a good/valid excuse? Not at all, but it happens.
I'm certainly not gonna complain about it, but it was a bad call. The umping overall in the game wasn't great, for either team.