Earlier quoted context omitted.
Except they use their own browser features, sync, telemetry, privacy practices, business model etc...
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.
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).