Shinseki WAS wrong about Iraq. The invasion and conquering didn’t require 500K, nor did beating Afghanistan. If he had his way, we would still be in the troop build-up phase.
The mistakes in Iraq were in handling things post-victory. Things like refusing to work with the Iraqi Army, etc. But if we waited for Shinseki to be ready, we would still be waiting.
Shinseki warned it would take several hundred thousand to take Iraq and to handle the lengthy post-war occupation, and all he was doing was quoting from the Central Command Op Plan that had been created the year before Shinseki became chairman. As it turns out he and the Op Plan were a whole lot closer to the mark than those who said the job could be done with fewer than 50,000 and the troops would be home in no time.