It would've been easy to keep the terrorists' identities off the radar by simply not advertising the fact that the they existed at all. But the Obama administration made "300 terrorists" a central part of their public relations campaign in advocating for KSM's civilian trial, so they should have been prepared for some scrutiny.
The 2009 Military Commission Act included an amendment by Sessions that stipulates that captured al Qaeda operatives are automatically deemed unprivileged enemy belligerents and are thus eligible for indefinite military detention. In other words: there's nothing that would stop the military from keeping KSM in Guantanamo until he met his maker. But Obama has pushed for the treatment of KSM as a civilian, and used everything in his power to justify that decision.
"So the question must be asked: how many of the 300 people on the list are also enemy belligerents that, if captured today, could be detained and tried in accordance with the 2009 Military Commissions Act?" asked Miller. "How many of the people on the list are directly involved in terrorism? How many are foreign agents of Al Qaeda? How many of them had trials before the military commissions were fully established? How many had civilian trials, like that of the Blind Sheikh, that were plagued with serious problems? These are crucial questions. Unfortunately, the administration is still not providing answers.”