Talk about unused land (without any further context as to why it's unused), BGW sits on roughly half the acreage SEAS owns (unless through the various corporate changes after AB they sold it) due to a conservation easement which makes it easier to get RPA buffer change approvals and potentially has old live munitions in the woods from the Camp Wallace grounds before the park existed. WC also sits on a smaller portion of their parcel, though I honestly have no idea what the extra land is for outside of there may have either been a requirement for a green belt when the park was originally developed or there were other development plans that never saw the light of day.
Then using KD as an example of unused CF acreage - per the other thread discussing it, it seems highly unlikely costs for using that land would be justified by demand so much that existing infrastructure including their boneyard storage and maintenance roads would have to all be removed or replaced... Plus the proposed county road and existing power lines would all have to be accounted for with any development plans.
Yeah, I'm sure in some of that land estimate there's truly usable space, but I'd guess it's not as much as that quote would lead anyone to believe.