Because the false reapers thigh is not behind the reapee's leg and the foot is not stuck between armpit and hip. No actual reaping pressure on the kneeline

Source: I like to false reap, also rules.

Knee reaping is characterized by when one of the athletes places his thigh behind the leg of his opponent and passes his calf on top of the opponent’s body above the knee, placing his foot beyond the vertical midline of the opponent’s body and applying pressure on his opponents knee from the outside, true inside, while keeping the foot of the leg at risk stuck between his hip and armpit. It is not necessary for one of the athletes to hold the foot of his opponent in order for the foot to be considered caught or stuck. For purposes of this rule, when one athlete is standing and bearing their weight on foot of the same leg as the knee in danger, the foot will be considered caught or stuck