Herb's blog post links to the SlashData Developer Nation Survey, so presumably that's what the claim is based on. The company has a methodology page here [1], and it looks like the Developer Nation panel [2] is one of the sources used by that company.

[0]: https://www.slashdata.co/research/developer-population

[1]: https://www.slashdata.co/company/methodology

[2]: https://developernation.net/