What I believe the author was trying to address in this framing was how the Black workers were pushed out later by (in his words) the company and the union.
I shared this except upstream in this thread but I'll use it again:
When most coal mining jobs were brutal hand-loading of the coal while working in two foot high tunnels, there were many jobs for Afrikan labor. But as unionization and mechanization raised the wages and improved the work, it became “too good” for Afrikans, and the companies and the UMW started pushing Afrikans out.
I really like the way D61 put it but I wonder if we shift it a little bit to be more specific that they could not be relied upon to have solidarity with their racialized union siblings.
It isn't relevant if they viewed themselves as proletariat, the union (and by extension the union members) allowed this to happen.