This is because governments will always try to pay for things with fiat promises rather than gold. It's Gresham's Law in action (aka "Good Money Drives Out Bad").
If I have 2000 fiat dollars and an oz of Gold, and I want to buy Oil (or Wheat or Rice etc) from you - I will always offer you the fiat dollars (the 'Bad Money' first). If you accept my dollars, that's great. But I would never lead with the Gold first.
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The Ruble Gold peg isn't the introduction of a Gold-backed currency. While people still accept Fiat, Gresham's Law forbids a fully exhangeable Gold-backed currency. However the peg does establish a floor to the Gold price.
This means that it's no longer possible (assuming efficient arbitrage) for the Gold price to be artificially suppressed below ₽5000 per gram.
Gold prices have long been suppressed/controlled by the issuance of synthetic (promissory, paper, IOU) Gold. That suppression mechanism should no longer work - as long as arbitrage is not somehow encumbered, and the Ruble maintains its current strength.
The arbitrage will only work if someone is buying/selling at the peg. Doesn't that make it a gold-backed currency?