The answer is buried right in the middle of the article:
Back in 1970, to win Department of Defense support at the program’s outset, NASA had redesigned the shuttle to launch national security payloads. Now, that decision paid off.
If you visit the Museum of the Air Force in Dayton Ohio, the guides will tell you straight out that the cargo bay (and thus entire airframe) of Shuttle was enlarged to be able to hold a Keyhole/CORONA imaging satellite and retrieve it if necessary.
Back in 1970, to win Department of Defense support at the program’s outset, NASA had redesigned the shuttle to launch national security payloads. Now, that decision paid off.
If you visit the Museum of the Air Force in Dayton Ohio, the guides will tell you straight out that the cargo bay (and thus entire airframe) of Shuttle was enlarged to be able to hold a Keyhole/CORONA imaging satellite and retrieve it if necessary.
https://en.wikipedia.org/wiki/CORONA_(satellite)