From what i heard is that it was def offensive PI but what made it unusual is that the ball was thrown towards Garcon instead of the player that was the benefit of the pick play. The SF RB was wide open if the QB threw it there and that would have been easier to see the OPI.
Its one of those doesn't always get called but happens quite a lot type plays. Did it save the skins ass? Yup. but if the earlier fumble wasn't allowed it might not have been the most critical play on Garcon.
Letting SF score on a long TD was worse IMO. That can't happen.