An iframe is basically a different window. That different window has its own URL. That window can have its own analytics, events from which will still be visible in your Chrome since when you open a window with an iframe in it, you're effectively opening two windows.
Your top-level GTM/GA can't see what happens in an iframe since, as I said before, it's a different window. GTM/GA are only responsible for the window they're loaded in.
If you think your iframe has tracking in it and you want to turn it off, disable it on the iframe page rather than on your top-level page.
For those who are wondering, you still can track what happens in an iframe with your external GTM, but it would imply elaborate implementation, i.e. the iframe would have to send messages to your window where GTM would be able to listen to ht