Historically there has usually been one currency as a de facto international currency, normally that of the largest economy of the era. The British pound was the most recent example prior to the dollar. It's not usually a matter of which currency is the "strongest," just the most prevalent. But more developed economies have usually used their own currencies for most transactions, especially if having regular trade with each other.
I wouldn't make too much out of this agreement. Russia and China should have been using their own exchanged currencies for awhile now, but one or both have always been too unstable for the purpose, hence use of a third party currency, which is why many other countries still do the same. All this represents is that in this case China is extending a hand of conciliation to Russia by agreeing to allow use of the recently wobbly ruble in transactions of greater importance. And there's also the matter of savings to both parties on FX charges. The article also noted that the two countries are ~ $100/mcm (1,000 cubic meters) apart on a price for Russian gas, the Chinese wanting that much of a discount vs. the price Europeans pay. (I think I've read ~ $300 vs. ~$200 elsewhere). That's pretty huge, a 33% reduction.
The Russians aren't doing themselves much of a favor being paid in rubles, on the surface, but any other currency has to be eventually converted to rubles anyway, and the big bonus here is that anyone would take rubles in payment for Russian purchases abroad. So in that regard Russia get's a better deal than otherwise on what it buys from China.
The only time to pay attention to any of these international currency discussions is if there were to be any credible talk of pricing oil in a currency other than the dollar.