This is the claim that needs substantiation, not that a key Wikipedia person supports Zionism or identifies as a Zionist, or that various employees or volunteers support Israel. Those other claims don't seem significant.
The opposite is also true - that various volunteers are antagonistic towards Israel. It's also likely that some employees are antagonistic towards Israel. You can find plenty of people's experiences discussing the same pattern you describe but with different political views - some antagonistic admin coming along and declaring that you lost the dispute.
Various "bosses" at Wikipedia have different views, ideologies and identities. Given that, what on Wikipedia can be trusted, and what can't be? What's special about views towards Israel?
Evidence is useful, but when viewing people as "adversaries", and if trying to "defeat" them in general, the game is already diminished or lost - for most everyone involved. Little productive conversation can occur.
> This is an issue that people on both sides are strongly emotionally engaged in.
Yes. But that is true of many issues in the world.
I'm not going to try to substantiate it; if you're not already aware that Wikimedia is owned and run by a self-confessed Zionist, you can look it up for yourself. I don't engage with the Zionists on Wikipedia; it's a waste of time, you can't win. Some Zionist admin will come along and declare that you lost the dispute. I'm not inclined to take up arms here either; it's against the rules, and anyway, in matters of this sort, you can't defeat your adversary by producing evidence or substantiation. This is an issue that people on both sides are strongly emotionally engaged in.