AGPL section 13 starts at the cutover, not at the announcement: the fork is
a modified AGPL program and its users reach it over a network. Settled
today that the source is released after the cutover, with the links live
then, and in the meantime the server's users are the operator's household,
so the people owed an offer and the people holding the repository are the
same people.
Anyone migrating their own server inherits that obligation on their first
day and has no such overlap, so the migration tool says so at the end of a
successful run instead of leaving it to be discovered.