I understand that the four tildes which generate the signature are used to signify that the revision is meant to be recognized by Echo as a notification. I could see how a user might want to correct a typo and would not want that showing up as a notification. I'm sure it could be overwhelming if every edit on that discussion page showed up as a notification.
That said, when you rely on using four tildes for this feature, it seems to require that the user's signature include a link to their user page. But if someone using MediaWiki with Echo chooses to use a signature that does not link to their user page, then Echo will not recognize those edits for notifications. An example of this: A user on SemanticMediawiki.org might set their signature to link to their user page on Mediawiki.org.
Similarly, I don't see why Echo requires a timestamp in the edit to qualify for notification. When the page is saved, the revision table is appended with an entry including rev_timestamp and rev_user. Why not just use that?
Why not just determine some minimum criteria based on number of characters added to the page or use some set of regular expressions to test that it was not just a typo correction?