It's a very plausible reason. I think he made a stink about it before hand to set the stage for claiming they were biased against him if he comes off looking bad. You have to think optics with these debates. It simply didn't matter if Quid Pro knew what he was talking about, he just looked pathetic, and that's why they forced him out.
I believe very little from politicians. You should try it, makes for a much more peaceful life with a lot fewer disappointments. And I still don't get why you're so desperate to move off the failed, "He's scared of her" nonsense. Here's a prediction, they're going to debate face to face, a certain percentage will claim before they even start that TRUMP! won, another percentage will claim that Harris won, some will complain that the moderators should have been harder on TRUMP! while others will say that they were too soft on Harris. Harris will make some verbal flubs that people will jump on, TRUMP! will make some grandiose statements that others will jump on. Have I missed anything? Oh, yes, the likelihood that she will dissolve into uncontrolled giggles after a nonsense word salad that answers nothing. That remains to be seen. I'm sure she's being prepped to avoid that at all costs, but hey, Quid Pro had a week off from his job to prepare and still zombied out, so there's that...
The bottom line remains, TRUMP! was never afraid to debate Harris. I don't think there's anyone he's afraid to get on stage with.