flight 99 is still CO
I asked an agent for an explanation. After checking with a supervisor, she said both the UA and CO websites say "Operated by United" because the HK airport authority only allows one operating certificate for the merged airline. This may be old news, but apparently there are a few different countries that have similar rules. In such cases the operating carrier is listed as United.
Although the website and presumably other things like tickets will say "operated by United", it's still a CO aircraft, check-in is with CO and, most importantly for Flyertalkers, the upgrades are handled by CO and according to CO rules.