Your analysis is essentially based on a 1 on 1 comparison. But those units are not equal, and thus cannot be compared, because they belong to different factions.
Riders of Higharden, belong in a faction with low access to high mobility. I haven't seen the unit card yet, but if their stats are indeed equal (6 inches move), they'll be the fastest Baratheon unit. Keep in mind Champions of the Stag moves 4 inches (12 inches total with march), which is as much as NW ranger hunters on foot. By comparison 6 inches cavalry will move 18 inches. Since Baratheon are usually slow, you are paying for the mobility at a premium in a faction that does not have access to it otherwise.
Arguably, mobility might not be that valuable, and Baratheons are overpriced compared to other factions - but that's a faction problem, not a unit problem.