Even Android had moved to a flatter design pattern 1-2 years before iOS. While Material Design wouldn't be released until 2014, you can see them moving in that direction from Gingerbread to Jelly Bean, particularly when looking at the system components and first-party apps, since this was before the concept of a unified design language across third-party apps had been formalized.

At the time Apple introduced their flat design in June 2013, they were the odd ones out. In fact, I remember a Daring Fireball article posted in spring 2013 (a few months before WWDC) praising Apple for leading the pack in flat design, and HN excoriating it for making what was at the time a clearly preposterous claim.