How did they get that idea?

Because it sounds good on paper and, if it was at all true, would totally 'own the cons.'

It's not like their movement is based on truth, but feelings. So, if you assume Their Truth™ is that The Whites are an oppressive group who throw their weight around and wield institutional power to keep their own positions despite being incompetent (retard colonialism), then it follows that pushing "DEI" would be pro-merit.

It's completely deranged, but you can sort of make the puzzle pieces fit, if you try to look at it from Leftist Reality™. If white dominance in a white majority country was unearned, DEI brings freedom and meritocracy. It's cultist logic.