Not true at all, and also nothing is stopping you from building your own ad-blocking logic in to your browser either - iCab Mobile for example does just that.
What's the technical issue?
Thats only used for user initiated navigation, not loading of assets.
You could still inject a JavaScript file or CSS file though to help hide pieces of the view. But that would be content blocking after the HTTP requests have been made.
I don’t know of a way to intercept the HTTP requests as they’re being made from a WKWebView like Safari does for content blocking.