Yes. That's right. There are some subtle differences between Medium and Substack mostly owing to the fact that Medium was originally designed as a blogging site and Substack was originally designed as an email newsletter platform (with a place to park blog posts as well). So each is a bit catered a bit more catered to their original goal, although they've certainly become more similar over time.

Ghost seems to be squarely in the middle: both a blog site builder/newsletter platform from the outset. Honestly, Ghost looks to maybe be doing the site builder piece even better (like more ability to remove Ghost branding relative to Medium/Substack, which makes sense since it's open source).