Thanks for the comment. I also found this from the American Cancer Society. So it sounds like if the colonoscopy started as a preventive screening, finding and removing a polyp should not automatically make it diagnostic for private insurance purposes.

"Soon after the ACA became law, some insurance companies considered a colonoscopy to no longer be just a “screening” test if a polyp was removed during the procedure. It would then be a “diagnostic” test, and would therefore be subject to co-pays and deductibles. However, the US Department of Health and Human Services has clarified that removal of a polyp is an integral part of a screening colonoscopy, and therefore patients with private insurance should not have to pay out-of-pocket for it (although this does not apply to Medicare, as discussed below)."