Rebbe Elazar, who still had Eidei Chodesh in his day, was careful to be Mekayem Lachem on Atzeres. Was this only when it worked out, or did he Pravve Isru Chag on those years? Why didn’t he say Vav Sivan instead of Atzeres?
As many Sefarim say, the Mattan Torah had more to do with Shaar Chamishim than it did with Vav Sivan. This is why it depends on the counting rather than the day. The Rivash didn’t come to throw everything we know overboard, as seems to be the style these days. He is discussing the fact that we celebrate Atzeres not by the date but by the count.
You can look in Tiferes Yisroel of the Maharal in Perek 25.