In the earliest versions of MySQL that supported subqueries IIRC you could see different EXPLAINs for what, to me, appeared to be equivalent queries. That is to say, I don't believe the query optimizer is/was MySQL's key feature at that time.
Then again, it wasn't that long ago (2003) that MySQL 4.0 came out. Before that the subquery format was a syntax error. :/
I would be very surprised if the optimizer did not treat his two example queries the same.