They control the identities, and if they wanted to they could transition the identities at the price of—at worst—data loss. It’s the difference between having a

They control the identities, and if they wanted to they could transition the identities at the price of—at worst—data loss. It’s the difference between having a blog using the Medium CMS and hosting and having one on a subdomain of medium.com—you can switch away in the former case but you’re completely stuck in the latter (which is why Medium took away the first option and Substack never had it).