@Anonymous This is reply to the message posted by Anonymous (with the capital A), so they should receive the notification. Does anonymous (with the lowercase a) also get a notification?
Ok, so the conclusion of the experiment is: 1) Typing @xyz pings all users where username begins with xyz. 2) If a user with username xyz posts a message, then direct reply to that message pings only this particular user. (Even if there is another pingable user with the same username.)
