If one uses Zionist as an ad hominem insult rather than more neutrally as a descriptor of that person's beliefs, of course this is anti-Semitic.
Imagine over 70% of African Americans were professed Baptists and you consistently use the term derogatorily, with the caveat 'I'm not a racist, I'm an anti-Baptist'. Exactly.
Though for some I'm guessing that this may require spelling out even more.
So, you can engage with or critique the tenets of Zionism or indeed Baptism. Why not?
What you ought not to do is straightforwardly use such terms hatefully against the people whose fundamental beliefs are encapsulated by them, and absolutely not as a camouflage for any deeper prejudices you might bear.
It really does not matter one bit that Israel or anyone else is somehow 'conflating' the groups Jew and Zionist. There is a default conflation which you really ought to acknowledge and adapt to.