it sounds like the ulcers in the rectum were deep as well as infected, and this might have prompted the preliminary speculation of CD instead of UC.
Until surgery, or critical choice between some biologic med options, CD vs UC does not matter so much because the initial treatment for each is the topical anti-inflammatory mesalamine - taken oral, rectal or both.
oral is not going to really reach into the rectum. Even after the ab handle the infection in the ulcerations, you still need to address the ulcers. this is best done with rectal meds.
As some of us Crohnies have said before, there is a difference in treatment between UC and Crohn's. The initial treatment for each is not topical anti-inflammatory mesalamine. Those are proven ineffective in Crohn's. They do not work for Crohn's. Yes, many GIs still prescribe them for Crohn's, but the data clearly show no benefit.
Having said that, I agree with the other posts questioning how the colorectal surgeon can determine it's Crohn's based on a look into your rectum only. The answer is, he can't.
I may have been out of date. I follow UC and only read about
CD when it is in the same article. I did try to confirm this in the literature, and I do see in a 2018 review that mesalamine is currently mentioned only for maintence in CD and not for first-line induction of remission.
Management of Crohn's disease has been seen as an evolving challenge owing to its widely heterogeneous manifestations, overlapping characteristics with other inflammatory disorders, often elusive extraintestinal manifestations and uncertain etiology. Therapeutic interventions are tailored to address symptomatic response and subsequent tolerance of the intervention. Chronology of treatment should favor treatment dose acute disease or "induction therapy", followed by maintenance of adequate response or remission, i.e. "maintenance therapy". The medications which are highly effective in inducing remission include steroids and Tumor Necrosis Factor (TNF) inhibitors. Medications used to maintain remission include 5-aminosalicyclic acid products, immunomodulators (Azathioprine, 6-mercaptopurine, methotrexate) and TNF inhibitors (infliximab, adalimumab, certolizumab and golimumab). Surgical interventions like bowel resection, stricturoplasty or drainage of abscess is required in up to two thirds of CD patients during their lifetime.