I don’t know anything about iOS development, but my guess is that since this is a google app they do show the chrome icon regardless if you have chrome installed or not. If you don’t have it, clicking that will take you to the App Store.
They show safari because that comes on an iPhone that that’s the default default browser (i guess that’s the way to say that?)
I don’t think they have a way to scan your phone for what other apps are installed and list the ones it thinks are web browsers here. What I mean is that I don’t think that it’s technically possible of feasible to display a Firefox icon here. How could it know you have that app?