I think they're specifically wondering if using @<username>@<instance> mention syntax will result in a notification popping up for the user on Lemmy.
I've been wondering that too (in the context of threads though) -- and if it does work, are there limitations regarding visibility between instances that people should be aware of. e.g. what happens if I @ someone in a post to a community on a lemmy server that is defederated from their home instance? Or, in a community that no one on their home server has subscribed to? Will they still get a notice?
I guess I don't really have a good mental model for how @ works on the Fediverse.