That is simply weak spin, Ed. Joe Biden challenged Trump to debate HIM, he did NOT challenge Trump to debate whatever chump the Dem party decided to throw into the lion's den. Yes, it was anticipated that it would be the two presidential candidates because both men at that time had control of the Electoral College delegates needed to clinch the nomination. The Democrat party and their hail Mary maneuver changed all of the details and arrangements.
Even with all that, you can bet that Trump with debate whoever the Dems end up with on the top of their ballot. And that JD Vance will gladly take on whoever the Dems draft for second position.
Do you think that either of the Dem nominees will have the guts to do the debate on Fox News? I say they will run for the cover of CNN or one of the other left wing media.
Edit: Here is the video of Joe's challenge to Trump. You will see that it is a direct challenge for HIM to debate Trump, not for anyone else in the Dem party to debate Trump.
Sorry for linking such a right wing biased site, but the same video would not load on cnn. I tried.
President Joe Biden issued a public challenge to Donald Trump, offering to debate him twice in the run-up to the 2024 election. Trump accepted.
www.rollingstone.com