That sounds inaccurate.

I opt out of the X-ray scanners (due to a family history of skin cancer) and have yet to encounter a problem. They just do a patdown instead.

What your ex may have misunderstood is that (if I understand correctly) if you don't explicitly say “I opt out” then you haven't actually opted out. So if I tell the TSA “I have a family history of skin cancer,” I haven't opted out… but if I say “I opt out” then I have opted out.

(I usually say “I need to opt out” just to be polite.)

The “red flag” language might be someone's second-hand explanation of that.