As much as I like bitcoin but it is not yet ready for mass adoption, the price of it is too unreliable for such a large long term commitment like what BRICS countries are making. Imagine if you buy a million barrels of oil (what oil tankers usually carry) worth $80 million in bitcoin and the price drops by 10% which is a normal thing in bitcoin. That is $8 million loss. That's not favorable.
Taking care of an individual capital and investment is more easily than doing it for a nation. I can stake my capital, accept risk and afford to lose it but honestly I can not do it with my national treasury. Especially if I don't host a public referendum for that decision. A national treasury can not be staked with a very volatile asset like Bitcoin.
However a centralized currency created and controlled by them is a lot more reliable because they dictate its value (exchange rate) so it can be kept fixed.
Governments can control their fiat currrencies, CBDCs and adjust their policies to support their currencies. They don't control inflation well but at least they will feel safer when they can control their currencies with their power and policies. With Bitcoin, they know that they can not control it because of its high decentralization.
At this point bitcoin is good for small size trades and as a small portion of the reserved assets of the country.
I agree with you that a small portion of a national treasury as a reserved asset is acceptable by assessing risk.