My interpretation was that Sterns and Jones were both going for the QB. Sterns stumbled, Jones tripped over him and ended up sliding headfirst into the QB's ankle/shin.
I looked up the rule, and I don't think it was applied correctly: "...no defensive player rushing unabated shall hit [the QB] forcibly at the knee area or below. The defensive player also may not initiate a roll or lunge and forcibly hit this opponent in the knee area or below.... It is not a foul if the defender is not rushing unabated or is blocked or fouled into this opponent.” (emphasis mine).
Jones stumbled over Sterns (so I don’t think he was “rushing unabated”), so my interpretation is that the flag should’ve been picked up.