You asked about a meaning of Zionist that they are missing, that should be seen as OK.
I'm not sure what that means. There are a variety of meanings and it is up to them to explain which one they mean.
As it stands they provide no definition so I can't judge what they are missing. Also, what do you mean by "OK"? Are you asking me for a definition which would make their decision to ban someone because of his affiliation with a group a reasonable course of action? If I am a scholar on Shakespeare and I want to speak in front of a group of law students about Hamlet and the legal implications in the play, are you looking for any definition of "Zionist" which would be an acceptable reason for them to ban me? I can't think of one.
This whole tangent started because - as I understand it - you were worried that an unqualified ban on "Zionists" would exclude speakers that shouldn't be excluded.
I've been trying to figure out what sort of people would fit in that category of wrongfully-excluded people in order to decide if you have a point with your objection.
So far, my feeling is that despite the fact that "Zionist" can be taken a few ways, I can't think of any ways to take it where my reaction to those people not being allowed to speak to a student club isn't "yeah - that's fine."
... so if you have some case where this
wouldn't be fine, I'm interested to hear what it might be.