No racial breakdown for countries other than the US and immigrants are generally not excluded in PISA.

Some of the countries, like Japan, are close enough to racially homogenous that the results wouldn't be affected. For France or something it would be more interesting to see racial breakdowns, if the data existed. I don't think that means the comparison in the chart is arbitrary

That data does exist but is mysteriously excluded. Perhaps because it doesn't support the race bait? Moreover, contrary to the figure's claims, PISA does not exclude immigrants. It's just a hodgepodge of crappy statistics. Don't get fooled by anonymous Reddit posters.