Case Book 9.6.1B is a stretch, because it involves A2 stepping out of bounds after an interception on the 30 and returns inbounds at the 25. Even though A2 was out of bounds for five yards, he is not guilty of Illegal Participation because A2 did not intentionally go out of bounds.
I would apply this ruling to the OP and since B’s missed tackle caused him to go out of bounds, he is not guilty of Illegal Participation.
9.6.1 SITUATION B: Third and 10 from B’s 40-yard line. A1’s forward pass is intercepted by B1 on B’s 20 and returned to midfield. End A2 accidentally steps on the sideline at B’s 30: (a) before, or (b) after the interception. In both cases A2 returns inbounds at B’s 25, but does not make any attempt to catch the ball or tackle B1. RULING: In (a), it is an illegal participation foul at B’s 25 because A2 returned after being out of bounds prior to the interception. No foul in (b) because A2 did not go out of bounds until after the change of possession with the interception by B1