I think you're trying to make something out of nothing.
"Where are the Welsh standing up and saying Corbyn is not a threat to them? "
You answer your own question. The picture was an answer to the same question: "Where are the Jews...." - as it was constantly being claimed on the media.
You'd doubtless see a Welsh response of this nature if the media and Welsh politicians kept trying to nurture the image of threats to the Welsh from Corbyn.
The picture is an entirely legit and called for response. It must be possible to allow responses to accusations of racism, without being accused of racism for it! Similarly, it's not Corbyn or his supporters who have raised the issue of 'identity'. Not to be able to answer the question raised without being accused of identity politics would be strange indeed.