Jbaldy22,
I think you are overstating the Rivash. The Rivash is saying that when we were still m’kadesh the Chodesh with witnesses it was possible that Shavuos would fall on the 5th of Sivan, and Chazal weren’t makpid to make sure Shavuos fell on the same day as Matan Torah.
He explains that now that our calendar is set they always coincide and therefore we can say “zman matan torahseinu.”
This doesn’t mean that they have “nothing do with” each other. It just means that it is more important to be m’kadesh b’r’iyah than to make it work out.