that would be the most logical explanation, but the article mentions that "The effect is smaller in Europe and especially large in India, but it does appear within each country, suggesting this isn’t the sole confounding factor." Which makes me think that there are other factors at play that add up.

Could well be. I know that Google for sure requires spaces, and some cursory Googling leads me to the believe that Facebook does as well. Does anyone have information on Amazon, Microsoft, Apple, etc.?

Of course, if all of the top companies are requiring spaces, then there's probably a reason for that, and its impact on salary data is thus meaningful, not just a spurious correlation.

When I was working in Windows about 5 years ago they required spaces.

Don't work there, but I know that Apple tends to use spaces for their public-facing code–I'd expect that their internal code does too.

It's sounding like all the top tech companies use space-based indentation. Our style guide makes some pretty convincing arguments as to why it's better. It sounds like good developers are either more likely to (a) work at top companies or (b) search out information on best practices for handling indentation. Mediocre developers probably just use whatever, which ends up being tabs a lot of the time.