I agree with the flight attendant. (Though maybe he/she might have been a little too harsh, but I wasn't there). I guess it also depends on how soon the takeoff roll would began once the aircraft gets into position. This something only the cockpit is privy to.
"...he should have been commended by the flight attendant for ensuring the safety of the other passengers at the risk of his own."
Well not exactly. Please bear in mind that when the passenger did this, he was risking the safety of SEVERAL passengers, not just his own. Simply put, if he was standing during a takeoff roll, and if the aircraft had to suddenly abort the takeoff, a standing passenger literally becomes a flying missile through the cabin, inevitably landing on one, two, three or more passengers. Necks could be broken, backs could be broken, and the guy himself could have been killed. Passengers have most certainly been killed this way on past UA flights. THIS IS A PRIMARY REASON FOR PAX (AS WELL AS F/A'S) TO REMAIN SEATED WITH THEIR SEATBELTS SECURELY FASTENED during takeoffs and landings.
During an aborted takeoff, a standing passenger is more likely to seriously injure (or kill) several fellow passengers, than a content from an overhead bin.
Again, the f/a was merely trying to perform his/her job and exercise leadership skills (albeit in a harsh way). He/she just "went by the book," and it's true that other f/a's may not have reacted the same, (although it depends on the takeoff/taxi timing situation). I have to admit though, that I've gotten up from my jumpseat, sprinted through the aisle to close an opened bin just before takeoff. Yes, this was risky, so don't tell anyone!