The extent to which a given seat can be reclined is adjustable by a mechanic. So, on the same airline, the same make & model of plane, and the same make & model of seat, the allowable recline can be different from one specific plane to the next. Indeed, the non-reclinable seats in front of exit rows are generally standard seats whose recline setting has been set to zero.

As for the height of the seat hinge: While seat manufacturers are trying to tweak this to optimize design within various parameters, the hinge height is typically not higher than the height of the back of your seat's bottom (i.e., the surface on which you sit) where that bottom meets the seatback. Often, the hinge height is substantially lower.

Recall, that seat bottoms are generally canted, with the back edge lower than the leading edge. So, even if the hinge and seat bottom are at the same height at the bottom's back edge, this inherently means that the leading edge of your seat bottom will be higher than the hinge. As one sits on such a seat, one's legs are higher than the seat bottom's leading edge and even more so relative to its back edge -- and, therefore, the hinge. An A=B=C sort of thing.

Also, consider that the height of a person's knees while seated may be determined only in part by the height of the seat bottom. For many people, when seated their feet touch the floor and thereby to some extent lift the backs of their knees above the height of the seat bottom's leading edge. In the extreme, picture an adult seated in a 1st grader's school chair. To a much lesser, but sill significant extent, this occurs on planes.

Frankly, I've seen this comment before about knee/hinge heights and it puzzles me. Beyond observing actual airplane seats, just picture a reclinable seat.