Trump is oblivious to the fact that BRICS is now a bigger economy than the G7. The US is no longer an essential part of the global economy.
Increasing trade outside of the dollar is the only way countries can protect themselves from economic coercion by the US. In particular, China can obviously see that the US will go after them the same way they did with Russia. So, it would be suicide to agree to tie their global trade to the dollar.
Meanwhile, China also happens to be a bigger trade partner than the US for most of the world. So if it comes to choosing between the US and China, it's not really much of a choice. Especially given that China exports things people actually need, while the US barely has any manufacturing industry left accounting for less than 15% of the overall economy. The US is a big market, but it's not essential the way China is.
As much as we might like to laugh at such statements, not being able to sell things to the US is still considered by many businesses and countries to be a big disadvantage. And since the tariffs don't appear to be against countries trading with BRICS, just BRICS members, they could theoretically not force countries to choose between BRICS
Going to the link, we see a website which doesn't actually go into detail about this "research", and the "short to medium term" is not defined in actual years, so could be anything.
Ah yes, tariffs, the famous ... counter-narcotics policy tool?