First of all, I doubt you read that entire article, because it goes on to say:

"Was he only referring to the clearly antisemitic meaning of “Zionists” in the rhetoric of SNCC militants? Or was he making a general statement? We will never know"

This is apparently an account of what someone heard MLK say at a dinner party or something, in a very specific context about 'SNCC militants' (whatever that is).

Anyways, MLK is no longer with us. If he were alive, he would likely be opposed to the war Israel is now waging, given how he was also opposed to the war in Vietnam - also a pointless, open-ended, unjust war. But, it's not possible to prove what he thinks or would think about Israel's actions today for sure, because he is no longer alive.

Many Jews in and outside of Israel criticize Zionism and/or the policies of the state of Israel as well. Are these Jews all 'anti-Semitic'?

Who decided that Zionists or people in Israel speak for all Jews? There's around 8 million Jews in the USA alone - many of whom oppose Israel's policies, and are critical of Zionism. I don't think you or any Zionist speaks for all of them.

Why can't Zionism be criticized? No country or religion or ideology is perfect. Saying we can't criticize Zionism sounds a lot like Islamic blasphemy laws.

I'm American, and I'm extremely critical of US policy too. Does this mean I'm against all the people that live in my country and wish them harm? Of course not, If anything, I want the US to stop bringing unnecessary death and harm on other people in the world - and this includes the death it is supporting by continuing to give bombs and weapons to Israel.