https://www.nytimes.com/2021/09/16/theater/sondheim-new-musical.html
My guess? It's a [title of show]-like meta-musical about the writing (and eventual abandonment) of Buñuel. The title is partly ironic since, far from "going back to square one" after the decision was made not to proceed with the earlier Ives collaboration, the new show recycles Buñuel's score and libretto (such as they exist) wholesale in the process of illustrating their creation.
Kidding, obviously (though almost nothing would surprise me at this point). I am desperately curious.
I suppose the "Buñuel" and "Square One" threads are somewhat mutually redundant at this point; and if the latter title was in fact extemporized on Colbert's couch, it's kind of hard to know just how (un?)seriously to take it. But for now I guess I'll go with it.
In case anyone missed this last week -- there's not a whole lot of real news here, but FWIW:
https://www.theatermania.com/broadway/news/cameron-mackintosh-stephen-sondheims-final-musical_94730.html