Leftists don't really want diversity themselves, this can be seen by the way leftists tend to live in ethnically homogeneous areas, although most of them still believe that diversity is a good thing and would still label anyone who criticised it as racist.
What proves this point is that the sort of places that leftists speak highly of and say how cultural and vibrant they are tend to be very ethnically homogeneous countries.
Leftists are most likely to say they much prefer Eastern European cities to Western European cities because they are so much more vibrant and interesting. The fact that these cities are almost entirely white doesn't seem to matter much to them.
You will find that it's cities like London, Paris, Rome, very ethnically mixed cities, that they would say they find the most boring and bland. It's Eastern Europe that is popular now with left wing travel journalists.
I wonder if even some white left wingers subconsciously want to live in an all white society too, but of course the UK is "diverse" so they aren't allowed to think that here, so they go to Eastern European countries where being white is the norm and they can enjoy being in an ethnically homogeneous society without being regarded as racist and bigoted.