By use of the word must, I believe this is an instruction to the franchisee of the Thameslink. Southern and Great Northern franchise - ie, they MUST be free to incorporate into the [East Midlands Railway] franchise when its lease for them starts in September 2021; if the transfer was subject to replacements being available, or if the 171s were to be sub-leased back to Southern, the EMA could easily have stated that
No, it means that in order for EMR to use them, the units must be available, ie GTR released them.
It is not an instruction to GTR to do that.