Having said that, in my experience, a lot of people have some vague sense of "Israel should not be a purely Jewish state, should allow equal status to everyone else, should give the right of return to all Palestinians [implicitly meaning that it would no longer be a Jewish majority state]" etc.

And most people don't have a good answer to the question: Poland is a state for ethnic Polish people, Spain is a place for Spanish people, Japan is a place for Japanese people; why can't there be a country that's for Jewish place? Why is a country for Jewish people somehow illegitimate?

(There are good answers given the history why this particular instantiation of Israel is a problem. So good that nobody has been able to solve these problems for so many years. But many people talking about this don't know those reasons - they just vaguely think "Israel bad, not allowing anyone equal rights is 'apartheid'" etc.)

Zionism at its core is the belief that the Jewish people have the right to self-determination in their ancestral homeland. This is precisely what "anti-Zionism" opposes. Middle Eastern/Arab "anti-Zionists" advocate for exactly what you mentioned in your second paragraph. Additionally, depending on their political affiliation, some may seek the subjugation, expulsion, or harm of the Jewish population in Israel/Palestine. The percentage of Palestinians advocating for a one-state solution with equal rights for all is at most 33%, according to the Palestinian Center for Policy and Survey Research (PSR).

Your "left-leaning" friends may not have expressed anything like that and might simply want to express solidarity with the perceived "weaker" side. However, by proclaiming to be "anti-Zionist," they are undoubtedly associating themselves with some aspects of hardcore Palestinian nationalism. It's puzzling to me how this is compatible with traditional left-leaning stances and why any self-described left-leaning person would want to be associated with that, except due to extreme ignorance about the meaning of the words and the actual situation on the ground.