You can acknowledge that Peter Mandelson may not identify as a Jew because only his dad was Jewish, and that other people who have Jewish dads do identify as patrilineal Jews - and also recognise it isn't appropriate to make comments about "counting gold" about either...

Also: If you don't identify as Jewish, don't start commenting on the legitimacy of patrilineal Jews

It is a very painful topic for people who identify as patrilineal Jews and shouldn't be casually debated in order to defend Len McCluskey's comments about Mandelson

And if you are Jewish and are commenting on the legitimacy of patrilineal Jews: your denomination of Judaism may not halachially recognise patrilineal Jews - but at least show some basic respect to the fact that theirs does, and be quiet