You are absolutely correct. The initial rationale for immediate surgery was to get surgical buy in. Its the same reason most of the initial bladder preservation studies used BID radiation: surgeons were understandably concerned we were delaying definitive treatment.
One of the best studies addressing the issue of timing is Stockholm 3 (BJS 97:580-587). Randomized people to 5x5 with immediate surgery, 5x5 with delayed surgery (4-8 weeks), or long course RT (no chemo, thats the big knock) with delayed surgery. Basically, all of the groups did well. pCR was similar in both delayed groups. The most important data isn't in the abstract. Roughly 1/3 of the immediate surgery patients had delays from end RT to surgery of 11-17 days and they did horrible. Basically a doubling of post-op and surgical complications. If you are going to do immediate surgery, it needs to be immediate (ie, before they hit peak inflammatory response). I do a lot of short course and the key is coordinating with surgery ahead of time. If the surgeon prefers immediate surgery, I don't start RT until they have a surgical date scheduled. Two of our surgeons will do immediate or delayed. The third anecdotally feels immediate surgery has more complications and only does delayed surgery.
There are also a good number of near total neoadjuvant studies that start with short course and then give 3-4 cycles of FOFLOX pre-surgery (giving an 8-12 week delay to surgery) and patients do fine.
As to the case in question; I am still confused as to what surgery they did. Did they leave the rectum in place or not? Even if they initially planned a sigmoidectomy, it is confusing to me why once they made the decision to remove the entire rectum they would do less than a TME this day in age. Technically observation is an option but if they got less than a TME their local recurrence risk is high and I would radiate. I also agree with most of the above posters. I have not done post-op 5 x 5. IMO, its probably not a great idea. Most of the post-op surprises I have treated have a lot of bowel in-field and I am not sure how well it would go. Are they going to give this person adjuvant chemo (realizing it is also controversial for T3N0 now)? If so, why not do that first and then finish up with radiation?