Corbyn has been loudly anti-Zionism for his entire career AFAIK. Also it literally did end his career (well, his leadership), the "antisemitism" bullshit is what other Labour members used to tank him in 2017 and he just didn't recover from there.

And there's a difference between "treading carefully" and basically throwing full support behind Israel (but asking them to be a little nicer) like Bernie is doing.