I really don't know that there is a definitive answer. There are people on both sides of the question that make statements with absolute certainty.
Sure, both sides have written a lot. But I have yet to see an example for two images from comparable cameras (same sensor size, roughly same generation sensor) where a difference in resolution results in a differennce in noticeable noise when viewed at the same size.
I personally find demonstrations through images to be fairly definitive.
If you compare images from different resolution sensors at 1:1, you're a) going to see more noise in the higher res sensor and b) doing it wrong

After all, the further you digitally zoom into an image, the more it's detail becomes apparent, along with the noise.
The truth is: keepers from R5 will be noisier on average, because you will be able to crop more.
That's only true, if you actually do crop more. If you don't, it doesn't hold. Then you just have a lot of high resolution keepers. But I guess the assumption is that you would go for 45 MP because you do in fact crop severely often (wildlife and so on), so then it makes sense.