That would make the 'White culture is under threat' the core message of your post, not "how multiculturalism is horrific". The post you made (and I replied to) talked primarily about criticism to multiculturalism, and white culture is only mentioned a single time.

Cultural globalisation merges all cultures into one common. Small (or less vocal) cultures get less representation in the global culture, while large ones get incorporated more wholesale. Cultural diversity in contrast try to keep cultures distinctive, while also trying to dodge cultural tensions. White supremacists generally require that cultures stay distinctive, but they don't care to decrease cultural tensions. That doesn't seems to fit well for either side, and makes attempts to do so to look as political attempt of associating white supremacists with criticism to multiculturalism.