Yes, the vast majority of people who claim to be anti-Zionism probably are anti-Semitic. (This has certainly been my experience.) But it’s still important to keep the two concepts distinct, or else you risk demonizing people for something they are not. And the more empty charges get flung around, the more dismissive people become of the real thing.
It’s also important to recognize that true anti-Zionism needn’t entail being anti-Israel. It’s perfectly possible to reject the principles of Zionism while remaining a strong supporter of Israel and its people.