What do you think about the “Northern cavalry” compared to traditional Knights? Luwin mentions them specifically as being distinct to the north’s small number of knights. Particularly how they manage to equal southern knights as heavy cavalry despite that they don’t come from a knightly class the would allow them to afford armor and horses and servants and the north does not hold tournaments for them to train. do they have a historical equivalent?

They come from a very similar class: “those lords and their sons and sworn swords.” So they afford armor and horses and servants because they are noblemen or they are in service to noblemen. And while they don’t do jousting tourneys, the North has a strong tradition of melees. 

As for historical equivalents – think of the Anglo-Saxon Housecarls up against Norman knights in the Battle of Hastings (although they were footsoldiers), or the Varangian Guard.