Biden has been conspicuously avoiding speaking at unscripted public encounters for quite a while now, though, and reading from an autocue at SotU is a far cry from having to react on the fly and put together coherent arguments in response to moderator questions and Trump’s lies during a debate. I have the feeling Biden’s staff knew full well that the debate was going to be rough going into it.
The thing is, Biden and his staff called this debate and agreed to its terms. Trump also behaved exactly as expected by avoiding policy and spouting off as much nonsense as possible, just as he did in 2020.
The conditions of the debate were exactly as Biden should have expected going into it, so the idea that he was caught off guard doesn’t seem likely.