Any tackle above waist height with foot, knee whatever would be a foul. Just because he stoops doesn’t make it legal. If he hadn’t stooped and been kneed in the chest, stomach, balls etc etc it would still be a foul. Match officials consider shin height to be a foul worthy of a red as evidenced by the recent Bruno debacle, surely chest height is the same.
Timing of the challenge is the only thing the officials can potentially hide behind, but a knee around chest height is still endangering and we all know (Rashford) that endangering is a foul and occasionally a red.