They did own up to it. In fact, it was one of the reasons they cited when it was announced that Firelands was going to be released for patch 4.2 instead of 4.1.
Originally Posted by Bashiok
(Blue Tracker / Official Forums)
[...] One of our biggest regrets in the pacing of Wrath of the Lich King was the Crusaders' Coliseum. The Trial of the Champion 10- and 25-player raid was released while many raiders were still busy with Ulduar, which meant a premature end to Ulduar raiding -- and a lot of players didn't get a chance to experience all of Ulduar's content during the normal expansion progression.
Even if they 'owned' up to it, knowing the fault and then proceeding to use that as your main argument is seriously flawed logic.