If I'm recalling correctly from chapter 4, this situation of inheriting land basically ended during the 1840s. But the white workers being non proletariat did not.

I think Sakai wanted to stress the class awareness of the white workers, and didn't want to confuse the issue of who is in the proletariat. That is, the slave labourers were class aware, very much so, and so comprised the proletariat. The white wage labourers considered themselves petit-bourgeois, and so were not class aware, therefore not part of the proletariat - even if, functionally, they should be.

Maybe Sakai just didn't like the term labour aristocracy for some reason - or, like in that really interesting dictionary you posted earlier, defined the labour aristocracy as part of the petit-bourgeois.