There is no browser engine diversity on iOS. Reskinned Safari doesn’t count.
Choice is what the individual (supposedly) has. Diversity is what actually exists across an ecosystem. iOS is the last bastion of browser diversity, because despite all this supposed "choice", it's pretty much all Chrome everywhere else.
https://github.com/minh-ton/reynard-browser
Real Gecko on iOS
Chromium on iOS can also be compiled with blink, using the ‘use_blink’ flag.