Are racist chants now acceptable on the British left?

On Friday the Guardian columnist and Corbyn-supporter Owen Jones sent out this Tweet to his followers:

Palestinians urgently need our solidarity. Join me protesting Trump’s Jerusalem speech outside London’s US Embassy *tonight* >> https://t.co/JfGW6sTqjJ pic.twitter.com/2VPeqf21og

— Owen Jones (@OwenJones84) December 8, 2017

As a video of the resulting demonstration shows, the crowd outside the embassy loudly chanted (among other things) ‘Khaybar Khaybar, ya yahud, Jaish Muhammad, sa yahud’. This is a famous Islamic battle-cry which might be translated, ‘Jews, remember Khaybar, the army of Muhammad is returning.’

https://twitter.com/hurryupharry/status/939253702497628161

The battle of Khaybar relates to a 7th century attack on a Jewish community by the armies of Mohammed.

Now two obvious questions arise. Why might this battle cry have been used on Friday night outside the American embassy in London? And are racist chants of this kind now acceptable on the British left?