Substack the company was founded in 2017 [1]. At that point Substack the person was already a household name in open source — with 76.000 GitHub stars to his name his projects' popularity were on par with the likes of Mozilla [2].

Naming an upstart company after a household name that isn't protected by copyright is incredibly clever. You get to use an established and trusted name, but can't get sued for using it.

[1]: https://www.crunchbase.com/organization/substack#section-ove...

[2]: https://web.archive.org/web/20170115065014/http://git-awards... — GitHub stars aren't a perfect metric, but it's the best I can come up with to convey the degree of name recognition back in 2017