Not similar. Significantly lower. In fact, it’s not even number one because Android is a lot less popular in the US.
On one major platform, there is Chrome by default (so even bigger market share issues). People can install other browsers, but far fewer do than even on desktop. For those that do, most use the Play Store which has some conflict of interest concerns.
On the other platform it's even worse. The only option is Safari and the conflict of interest is fully realized because other browsers aren't even allowed on the App Store
At one point Samsung came with both Samsung Browser (chromium based) and Chrome preinstalled, but I'm not sure if that's changed in recent years.
Chrome dominates because it's better. People install it.
This is such a persistent myth on HN - I think I read it at least once a week - usually on one of the many alternative browsers, plenty of which are available on the AppStore.
WebKit is the building block of browsers on iOS, but there are an extremely varied selection of browsers using it.