That’s it. Comments (stories in the API) re a separate object from tasks; you can’t get them both in one API call.
While I haven’t tested this myself, based on this list of properties from the Asana team I think you’re right that editing a comment doesn’t update modified_since:
This is also consistent with the documentation for modified_since which says:
Actions that count as modifying the task
include assigning, renaming, completing, and adding stories.
The boldface is mine - note it says “adding” stories and doesn’t mention editing them.