Well, /filker -k isn't calling the alias asynchronously, /filter is more like /scid rather than /timer here, /timer isn't blocking the actual routine when executed.
Even if you call that alias a 'callback' alias, it is still called in the same chain as /filter, the same way an alias calling another alias is and it's not because others parts of mIRC use some callback alias method asynchronously that it apply here.
