It is a bilateral relationship that started decades ago. Also keep in mind that the two countries are neighbors sharing a border. Bitcoin is definitely not the ONLY choice or even the best choice at this point (mainly due to high volatility) but I am certain that at least bitcoin was considered and I am fairly certain that it is one of the choices and it is being used today. One reason I have is that an emerging super power called Iran has strong relations with both nations with a huge trade volume between them and Iran has been using bitcoin under the radar for at least 3 years now.
Of course this is partly speculation but based on strong facts.
P.S. The transition from dollar (de-dollarisation) is happening. In 2015 90% of trades between China and Russia was done using USD while in 2020 it was only 40%. Russia is also significantly reducing their USD reserves.
If this decision is really agreed, it will lead to BTC, which was created from the start to be free from the centralization of any party. This new medium of exchange answers the challenge for those who are constrained by existing regulations and even get sanctions based on the interests of a group. The US is very powerful over its USD while other countries use it as a medium of exchange between countries. BTC has made it easy for the solution to their problem, if there was another way it would take more time.