The name "Substack" was co-opted from James "Substack" Halliday [1][2], a well-known hacker in the Node.js community. It wasn't an indie-brand-turned-corporate; it's a person.
The way substack.com has used VC money to take over someone's identity and feed off their reputation is incredibly scummy.
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
I view it simply as a catchy name and nothing more.