I thought of this when I was looking through the implementation of InterceptNavigationDelegateImpl#showIgnoreNavigation() in order to respond to your earlier question

I tried it and we don't open CCT for URLs in iframes. Have you seen the case of us doing so?

Ideally I would add a test for it, but there is a technical issue. All pages I serve from the test server are in scope of the webapp, and all big pages I can rely on (e.g. google.com) have X-Frame-Options set to disable iframing. Therefore it's hard to perform off-origin navigation in an iframe in test. This will be easier once we roll out WebAPKs which have more fine grained scope.

Sorry for sending you down a rabbit hole. I was wrong.

InterceptNavigationDelegateImpl#shouldIgnoreNavigation() only gets called for non http <iframe> navigations. Those get filtered out in WebappInterceptNavigationDelegate via UrlUtilities#isValidForIntentFallbackNavigation()