As we all know, stills are rubbish for this kind of thing. If you see the angle
@JamesL has posted but in motion rather than still, it's clear that the contact is with the Arsenal player's chest initially, his arm then slides up in the process of trying to lever him off.
He can't use his palm because of the angle the other player is at, you'd need to dislocate your shoulder to manage that. Yes, maybe he could have used the back of an open hand, but it's fairly obvious that you can generate more strength by clenching your fist at that point. It's a valid point if the contact is led by the hand/fist, but where it's led by the forearm, the closed/open fist guideline is misleading.