Why would AGPL have made it a no-go for Substack? Also, it looks like Ghost wants to collab but there's nothing from Substack in the Twitter thread you linekd?

IANAL but I think integrating AGPL code would affect the license of the rest of the codebase too: Substack would be forced to release the rest of their codebase as AGPL too. This is why AGPL is considered a "viral" license.

Because AGPL infects their entire service. You'll be hard pressed to find any commercial service adopting AGPL projects. Even the GPLv3 is often immiscible with commercial services. At least GPLv3 can be contained to only part of your stack.