Hence "studio" recording, not "original cast recording"..

in the biz, the term studio recording refers to any professionally produced (non-bootleg) recording made with the *consent* of the copyright owner, but not necessarily the express endorsement.
In this case it's a studio concept recording produced and sent out to try and convince backers to part with their money..
Other examples of this include the Les Miserables original French concept album, the Chess concept album..
The OLC of Evita did indeed feature Elaine Paige, who was offered the role as a second-choice, after Julie Covington turned down the role of Evita on stage to focus on her (very) private life.
I think ALW is on record as saying he wished Covington had stayed on..