There is a recency bias that comes with rating presidents and we are in one of the most polarized periods in American history right now. It’s not that Gallup is wrong, it’s that there is a lot more context to consider here. A president with this approval 20 years ago was dead in the water. But it’s not 2004.
Let me very clear that this does not mean Biden is in a good position. I could never argue that in good faith. He’s got a hard election ahead of him. But a lot of these people who disapprove of Biden either hate Trump more or already hated whoever the Democrat was going to be regardless.