It makes sense, though. The push to make the televised leaders’ debates Tory and Labour only comes primarily from Labour. Starmer will barely have to open his mouth if he’s facing Sunak- just let Rishi be Rishi and Tory votes will keep falling.
But throw a Green, or even a Libdem, into the mix, and viewers will be reminded how far Starmer is from what the country really needs. It makes sense for him to chicken out of that.