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

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.