Heroes 5.5 WarGame Edition - Tactical Edition.

knight, the second-to-last tier unit; its elite version is almost as powerful as angels.

Firstly, at tier 7, we have not only angels but also, for example, dragons. Therefore, in your case, it turns out that the knight is almost equal to a dragon, which is absurd.
But again, in this case, it turns out that we are equating a dragon to an angel, which is not very good, although it is more acceptable (in this mod, the increase in dragons is twice as low as the increase in angels, meaning 1 dragon = 2 angels).
Besides, even the comparison of an angel and a knight does not allow for the words "almost equal" - a person on a horse remains a person, who differs from an ordinary armored soldier only in the presence of a horse, and possibly in combat experience. All this can only result in the following bonuses for a person who has become a knight:
- increased speed
- the ability to deal more damage due to the charge
I don't see any reason why a person on a horse should have many times more HP than a regular person.
The only thing that can explain the position of a humanoid unit at a tier higher than the fourth is a connection with magic, which cannot be explained from a physics perspective.