I have little experience with native iOS development, so you might well be right. That said, Telegram strikes me as quite a bit simpler than Slack. Not just in number of features, but also considering how many of the features are 'strange' UI.
As an aside, apparently the next version of Telegram will be a complete rewrite done in Swift, and snappier as a result.
Windows and Linux are another beast, and Telegram’s solution is to use Qt for those platforms. Again, slack like capabilities with no webviews.
The difficulty of native dev is not as high as it’s often represented.