This is an irrelevant argument in a perspective of monopoly and its market definition. Even Apple itself gives a list of competing web browsers in the iOS ecosy

This is an irrelevant argument in a perspective of monopoly and its market definition. Even Apple itself gives a list of competing web browsers in the iOS ecosystem to refute App Store monopoly arguments, but this also strongly suggests that iOS and Safari are separate products and browsers in iOS itself is a competing market. The only possible argument is that iOS is not in a dominant position in the smartphone market thus Apple cannot exercise monopolistic powers. But people doesn't change their phone because of web browser engine since its prohibitively expensive for this purpose. The result is that an Android phone doesn't work as an alternative for iPhone, thus iOS can be defined as a sole market for browser products (in terms of monopoly of course). This could've been solved if Apple allowed installation of other OS in iPhone, but they've made their choice a long time ago.