Snopes has always leaned slightly to the left. If the urban legend they're investigating is political, they'll give a little more weight to the left than the right.
If they had done that after that garbage movie by Moore came out, it would have helped them.
Screaming, yelling and calling Moore a liar, might have felt good for them, but Scopes was embarrassed by the whole thing. There is a partial apology to their behavior towards Moore.
However, based on that, I can't say I'd call them left leaning, but I do concede its possible they overcompensate.