I actually think they thought he would. Not debating without a reasonable excuse would have gotten him clobbered. So, he offered Trump debates 100% on Biden’s terms. If Trump pushed back on anything, Biden would have his excuse to walk away. “We were ready to debate but...” Whether Biden’s donors would have been satisfied with that, who only knows. He’d at least have a plausible excuse.
Of course, doing it this way keeps RFK JR off the stage who would have otherwise qualified for the officially sanctioned debates. Biden’s #1 attack will be on abortion, but the issue becomes muddied if the third guy on the stage is just as radical as he is on the issue.