'Think' in a sentence of that type generally doesn't mean 'know', especially when prefixed by the 'it's difficult'. You're taking it as 'it's an easy decision, it was a definite penalty' - but he didn't say that did he?
Also, if you're convinced it was a penalty - was it because of this...
Erm - that's not quite what he said is it?
https://www.sportskeeda.com/amp/football/clear-penalty-xhaka-admits-to-zaha-foul The Switzerland international initially protested to referee Martin Atkinson but accepted he was in the wrong afterwards. "It's difficult. I think it's a clear penalty,"...
His foot doesn't get clipped as such. Yes, Xhaka may have touched the front of his foot - but then Zaha pulls his foot back, puts his feet together, and then bunny hops.
But that initial tiny bit of contact is why he won't get a ban. It can indeed be both a penalty and a dive.
Optical illusion I think. But agree with Sheebo about the spring Zaha puts into it, the only contact was after his little kangaroo leap. 4.15 on the bottom set of highlights here shows it a number of times https://www.ngolos.com/videos/2018-10-28-crystalpalace-arsenal