Claiming that there was a secret Jewish conspiracy to commit 9/11, and they secretly warned all Jews to stay home, is antisemitic. Sorry to tell you that, but it is.
Saying maybe Israel knew, and did not tell anyone, is wrong, but might not be antisemitic. Why would Israel magically know things that America did not? Al Qaeda was not an immediate threat to Israel, so was not a priority.
I really do not think Israel benefited from 9/11. It killed the peace process in Israel, which might be considered a benefit, but might not. It destabilized Iraq, and now Syria, which means that Israel no longer has a single nation state on its Syrian border, but rather a collection of independent actors. Iran is much stronger, and much more aggressive against Israel.
But I will concede that some in Israel may have thought they would benefit from 9/11, had they known about it before hand.