Just checked the timelines involved in that Xhaka tweet. Arteta joined City on July 3rd 2016, Xhaka joined Arsenal on May 25th 2016.
https://en.wikipedia.org/wiki/Mikel_Arteta#Coaching_career
https://en.wikipedia.org/wiki/Granit_Xhaka#Arsenal
Probably had the deal sealed with us/Pep unofficially before then.