At first I actually wasn't sure if it was a stamp, but now I've watched it again a few times, I think it is.
Few factors
1). Intent. There were seconds left for Sunderland to avoid a defeat, and then Arteta went over very softly due to a touch from Sessegnon to waste a few more seconds of time. There is a big, big reason for Sessegnon to be really pissed off at Arteta at that point, so we have the intent.
2). Once Arteta has fallen, Sessegnon is standing still for at least half a second. He then proceeds to build up momentum to step over Arteta and stamp down on his leg in the process. It wasn't like Sessegnon was already moving with a lot of momentum. He was stood still about 2 feet away, so why hop up and bring your leg down with so much force?
3). Look at the angle. The other player is what you would call an attempt to avoid stamping on him, and he was in full movement. Sessegnon was stood still, went into the jump, and then brought his leg down at an acute angle despite their being 2 to 3 feet of space between Arteta and the ball at that point.
4). Acceleration. Watch when Sessegnon's leg comes down. He actually speeds up his leg just before the moment of contact, which is exactly what you would do in a stamp.
Sorry, but to me that was a stamp. All the signs are there. In a moment of blind frustration due to what was pretty much a dive from Arteta in those dying seconds, Sessegnon stamped his foot down on Arteta's leg whilst retrieving the ball. It should be a 3 game ban.