Looks like the reality is that Biden won the debate.
An opinion poll would be expected to say just that. It's close to matching the opinions of their respective % of followers.
While it may be true that some (maybe even most) voters will view their preferred candidate more favorably than the opposition, it is not universal.
After the first presidential debate, only roughly 33% of voters said Trump had done a good job, despite the fact that at the time Trump's polling numbers were in the low 40% range. That means that roughly a fifth of his own supporters said he did poorly.
The same thing happened in the 2016 election, with polls regularly showing people thought Clinton did better than Trump, even more than the popular vote polls were showing.
Trump is currently sitting at 42% in the polls. The CNN debate poll shows him at 39%. That suggests that a significant number of his supporters said he lost the debate too. (It might not change their mind and they may still vote for him anyways, since they may not see the debates as critical in forming their opinions.)
If Trump really had "won" the debate, you would expect the post-debate poll to show him at least equal or even higher than his general poll standings.
https://projects.fivethirtyeight.com/trump-biden-debate-poll/https://en.wikipedia.org/wiki/2016_United_States_presidential_debates