It was a penalty, according to current interpretation of the rules, he clearly stands on his foot and takes him down. First angle makes it look like a dive, but other angles it is clearer, however personally I am not sure it should be enough for a penalty, two players fighting for the same space put their feet into the same spot, one just a brief second after the other, it is harsh IMO, but there are many times I have seen what I considered accidental incidents in matches and the ex pros commentating state that the player committing the foul knew exactly what they was doing.
It is though, I think only the second pen given against the refs initial judgment that I have seen in the PL (the first was in the manchester derby) hopefully it is a change in the use of var to make sure it is the right decision rather than the trying to judge just how badly a ref got it wrong before deciding to overturn his decision. As long as they now stick to this, I think it is better than saying, “well if he is a bit wrong, just wrong or even very wrong, we will let the decision stand, but if the ref gets it very very wrong then we will consider overturning it” which seemed to be the system for most of the season so far.