there are those who say that historically, yiru eineinu was instituted to help out the latecomers come for tefillah b”tzibur. Since the Gaon eliminated this Ashkenazic practice, it is unlikely that he was concerned with latecomers, at least according to this explanation.
Interesting, but not at all muchrach. There are latecomers and there are latecomers. It takes a minute of tardiness to miss Borchu, but it takes a few to say Yir’u Eineinu.
There are other possible chillukim as well.