Yes, but is that ethnic Russians? The birthrate in France is up, too, but how much of that is owing to Muslims?
I have heard that Muslims are now almost a majority in the Russian military, and that at some point not too long from now they will be a majority.
However, I think that what is overlooked is that even today, religion is secondary in Russia. Russians will say they are "Orthodox", but this is more an affinity to culture, rather than religion. That may change, but it is the reality today. A Tatar, for example, may be Muslim, but will usually identify strongly with being "Russian".
This is not dissimilar to the past. Ukrainians within the Russian Empire considered themselves citizens of the Russian Empire. And how about the millions of Germans? Or the Scots who landed in Russia after the Battle of Culloden (Russia's most famous poet had a Scots surname). Or the Irish? (where do you think the name "Korbut" comes from?