True, but the counter to that is what else do you have to go on?
What, me, personally? Not a lot - but then, while I am a member of the Labour party, I'm not closely involved on a daily basis, and I sure as hell don't spend my spare time trawling through pro-Corbyn Facebook groups. Moreover, I'm not Jewish. But I've read enough remarks and articles from people who
are that closely involved, and who
are Jewish, to the effect that if they reported every nasty comment or dodgy graphic they saw, they'd never do anything else. So, with respect, your comment sounds a bit like someone saying "How do you know sexual harassment is common?", just because women don't usually go to the police over being catcalled by builders or getting their bums grabbed in a nightclub. I'm not personally and directly affected by this, but that isn't a good reason to adopt a default attitude of cynicism towards those who are affected.
More broadly though, this just illustrates an important aspect of the whole problem, which is that antisemitism is the only kind of racism whereby people on the left, including many who consider themselves paragons of anti-racism, but who are
not members of the affected ethnic group, feel qualified to deny there's a problem, or insist it's "blown out of all proportion", or is the result of "smears" by political enemies, or even that it's a media con job being perpetrated by the very people who are being affected. It is never,
ever enough just for Jews to say "Look, there's a problem here" - there are always demands for proof, which are never enough, and then we get this weird "bad apples" calculus showing that, since most Labour members haven't had a formal complaint lodged against them, everything is therefore fine. Whereas I'm sure you'd agree that there is tons of Islamophobia in the Tory party - right? - but you're obviously never going to seen an article in Jacobin saying "Well
hackshally only 0.1% of Tory party members have had complaints made against them, so it's all a fuss over nothing, orchestrated by their enemies."
To put it another way, if 85% of black people in Britain said that a certain politician had an established track record of saying and doing things that made them think he had a fundamental anti-black prejudice, would you feel comfortable in dismissing it or saying that it was massively exaggerated? No matter how much you personally liked that politician?