Is workplace racial diversity negatively correlated with unionization?

That’s seems to be Amazon’s assumption in leaked documents about Whole Foods. According to Business Insider:

Store-risk metrics include average store compensation, average total store sales, and a “diversity index” that represents the racial and ethnic diversity of every store. Stores at higher risk of unionizing have lower diversity and lower employee compensation, as well as higher total store sales and higher rates of workers’ compensation claims, according to the documents.

Could explain the spate of unionization in online media.