I kind of see this as the whole "Substack is trying to become Twitter before Twitter becomes Substack" kind of race. Twitter added long tweets and subscriptions, if you could do markdown formatting and inline images in your long tweets - why would Substack authors stay on Substack when they could post basically the same thing to Twitter and have more audience (or potential audience) exposure.

If Substack sees the above as an existential risk - which it might be if Twitter executes well, then Substack is replying by trying to do the reverse to Twitter.