> I get that Matt based WordPress on open source software initially, but 99% of the work that became what WordPress (and by extension, WP Engine) is today was done by him and his company.

Wordpress became as successful as it did because of the open-source license.

If you were starting a website of your own using a tool just like this, and it wasn’t up on GitHub with a fully open source license, would you use it or look for an alternative that met those criteria?

Wordpress extracted significant value from the open-source license itself (and probably wouldn’t exist today without it). I’m not sure they realise that.