That's a fair point and it should be taken into account. But at the same time, this is Nate Silver. He's essentially the pre-eminent expert on polling, polling errors, and best practices in that regard. And what's more; when you imagine the different potential political factions amongst the African American community (a practice Dems try not to do); it's not hard to see why 2016 to today could have soured them significantly. Some examples from my family, friends, and extended family:
As much as Republicans aren't great on "black issues" under Trump Black Labor saw unprecedented gains in employment and income (until COVID hit). And a cohort of black voters are economic voters first.
The "vaccine mandate" talk from the left didn't go over well in Southern Communities where the oldest (like my grandparents) remember. And Oftentimes knew people that were part of things like Tuskegee (which was also one of many experiments like this. It's just that the exceptional journalistic work and integrity of several academics and journalists brought this one instance to light).
Student Loan debt affects people from lower and middle-class families a lot. The promise and almost delivery of student loan relief and then the total rug pull of it and sellout of the Biden Administration towards it soured lots of people.
It's not like Africans don't know what Alzheimer's is. And Biden's refusal to even address the concerns impact black voters as much as they do whites.
For the BLM cohort; running the guy who designed and championed the three strikes policy that has put so many blacks in jail for life unnecessarily and then running a prosecutor who knowingly tried to keep innocent black people in jail to maintain appearances doesn't sit right. Especially when the other guy made it a habit to pardon wrongly or likely wrongly convicted men and women of color and did so at an exceptional clip.
Inflation is super high and FED interest rates are 59 times higher than they were when Biden was elected. For "low information" voters that's just Biden's fault. But even for "high information" voters they realize that Trump was willing to bully the FED to keep those numbers down. "Stagflation" killed Carter it can kill Biden too.
It's really not at all surprising that the current Dem ticket isn't going to win black voters at 90% clips.