I think managers generally aren't capitalists. In that they don't have sufficient ownership.
I find it convincing that there's not just two useful economic classes for activists to consider, but three. (Each class having areas of antagonism with the others, etc.)
In the case of mom 'n pop stores, I can imagine (but I don't know) that in the old days, relatively minor people at the court could end up exerting much more power than many a feudal lord.
In slave societies, those supervising the slaves were often of the same race as the slaves, but better compensated. Yet they weren't of the same economic class as slaveowners.
Of course, the number of classes depends on our interests. So for instance, many sensibly observe the US as a monolithic economic powerhouse. Whereas marketing professionals might distinguish zillions of classes.
Tayssir