I don't think you understood the linked thread. The problem is not that they are using Ghost. The problem is they are not including the attribution required by the license.
Hey, Substack CTO here. We don't distribute Ghost's code at all and the only piece of code included is a client-side search library used by the third party theme. But that library is actually hotlinked and hosted on jsdelivr (via npm) with no modifications made to it what-so-ever. This includes the line at the top with the license link as Ghost originally built it
I understood that criticism but don't think it's correct, as many others have pointed out. This is MIT licensed software running on a server. Substack is not redistributing software to end users, by any common definition.
As shown in the thread they are shipping Ghost's client side code to the browser. The attribution needs to be there.