I agree Conan. There was definitely contact, his ankle was clipped. You don't fall over from a clipped ankle at the point of the clipping contact - you fall over then that leg then hits your other leg which happens 1/2 - 1 1/2 strides later.
If his ankle was not clipped, he would not have gone down.
If he had not gone down he was through.
Penalty.
The only two possible counter arguments are (a) there was no contact, he faked it and dived (which I don't think is the case watching the replays) or (b) that it was unintentional on the part of the defender...
I think it may well have been unintentional... but I don't think it matters. I think the contact brought him down in full flight, intentional or not, it's a penalty.