I don’t care for the company, but give Substack credit for clever marketing. It’s somehow persuaded a lot of people that newsletters should be called “Substacks” — when they’re just one of many newsletter offerings, albeit a popular one.
No one calls anyone else’s blog a “WordPress” — even though the vast majority of blogs are written on that platform.
It’s understandable that people who publish on Substack do this.
But if you aren’t and do it anyway, you’re doing free PR for the company.