He's likely talking towards the undecided and unregistered voters. Yes, we all know Black men and women who are registered will vote over 85-90% respectfully for the Dems, but since 2016, elections have been a bit unusually close, which means that the more newly registered or participating Black voters actively involved, the better chances of winning.
People complain about the last election in 2020 and this one potentially being too close but fail to understand that it's only close because there are not enough active voters to skew the balance
in favor of one side.
That 85-90% of Black (men and women) voters voting Dem does not represent the entire Black population of potentially eligible voters. Obama is trying to encourage more Black people, men in particular who might not be active or undecided to support Kamala in November.
This isn't about the ones who are already surefire to vote come November; this is about the undecided, unregistered, and dormant (registered but inactive for whatever reason) voters to come out on November 5th.