All of those things are ancillary except for privacy (even though privacy capability is closely tied to the browser engine these days). It's like saying it must be a Cadillac... "but I can choose the seat colours and stereo so that counts for something!" - i mean sure if you value the stereo that much but you have no choice over the things that makes it what it is, the engine.
DeLorean DMC-12 and Renault 25 share the same engine. By your analogy these would basically be the same cars. Which of course is apsurd.
Engine just makes the car move, everything else defines it. Same with browsers on iOS. Also saying Chrome and Safari on iOS are same browsers because they share WebKit is like saying Edge and Vivaldi are same browsers because they share Blink (they share much more, but still they are very different).
You can pull my analogy to pieces all you want, but it doesn't change the fact that you are using apple-webkit, when you use any browser on iOS, not gecko, not blink... and unlike a car, the browser engine makes up 99% of what the browser does. but if you want to argue that bookmark syncing and shit is a significant feature, i mean sure whatever, i think you need to open your eyes.
