I don't think squashed commits matter since they used project byte size as a filter to get rid of "Hello World" sized noise.

Hard to tell how much private repos would sway the results. Maybe a large number of COBOL programmers are stuck behind their organization's private repos and all we see are the languages they play with on the side for fun?

I think it could make a pretty large difference, actually. Languages like JavaScript (I know it's not included here, but still), Ruby and Python have large open source communities creating time of libraries in common usage. Compare that with a language like C#, which has an extensive standard library and comparatively less open source third party libraries. You'd expect that even if there was an equivalent number of private repos in both cases, the languages with a larger open source ecosystem would appear more popular in this analysis.

My private repo usage is heavily C#