Totally fair question. Substack is great for what it is, but it's a newsletter. Your posts show up in reverse chronological order and a new subscriber sees whatever you published most recently, not the thing they should start with. Lesso is for when you want someone to start at lesson 1, work through to lesson 6, and learn something in a specific order. A Substack archive and a structured course are pretty different experiences for the reader, even if the writing is the same.

They're not really competing. The use case I'm thinking of is writers who keep their Substack for audience building and use Lesso to package their best stuff into something they can charge for separately. There's a Substack import that pulls your posts in so you're not rewriting anything if you want to just repackage what you have in Substack onto Lesso.