Why would Russia buy a bunch of gold and then buy more gold for rubles? Why would that make anyone want to hold rubles? Isn't that the purpose of the plan?
Russian Oil (and the historic devaluation of Russian assets) might make people want to buy Rubles. But I think the purpose of this floor on the Gold price really is to support the price of Gold, to switch off/damp down suppression and perhaps to force a Gold short-squeeze.
The ratio of synthetic to real Gold is at least 100 to 1. Probably way more.
When reality comes to the Gold price (and it goes way up) then all fiat currencies - and all declared reserves of Gold - will come under extreme scrutiny. Russia, China and India can certainly withstand scrutiny of their Gold reserves.