Can diverticulosis ( not itis) be mistaken for colon cancer on ct scan? pcp says no. no referal for other tests.

No, not really: Cancer will typically produce a growth which has obvious size and dimensions. Diverticulosis does not produce a mass or lump unless it is inflamed and infected in which case it would be called(as you said) diverticulitis.

Possible but rare: Yes diverticulosis is benign , has distinct appearance from CA on CT for a trained radiologist will read as accurate as reading finger prints , some times in between diverticula the wall may be thickened , may have mucosal overgrowth will mimic a early cancer , then colonoscopy indicated , at 39 cancer is not common but age is not exception for CA , ask again for referral .