i think what he means by propensity is where a certain demographic falls on the likeliness to vote spectrum. Kamala doing better with older white men and women is a good thing because they have a high propensity to show up and vote. Even if trump carries that demographic, these types of gains are good when some counties will be determine by a few thousand votes. On the other hand, for black men, where is trump making his gains? black men as a whole vote less than other groups, but the ones that do vote, generally vote consistently and overwhelming vote democratic. Is trump making gains in favorability with black men then are less likely to vote anyway or in the demographic of black men that do vote consistently? My guess is the former and not the latter but we won't know for sure until exit polls come out.