So you actually believed that the volatility of BTC can be the same as the fiats? I think that is unlikely. Even though both of them are currencies but it is just that the other is unique. BTC has a fixed and has a small amount of supply than the fiats, which is the reason why it is volatility is high. Also, he didn't underestimate the volatility of most currencies in the world but he is only comparing there. In fact, he thinks they are much better in terms of stability.
I think none is forced to use a currency even if that currency is from our own country, but it is just that our local currency is much easier to use and then again they are very stable. Another thing is like you said, people may lack in knowledge to check the other.
If talking about assumptions maybe everyone can say something based on their knowledge but if talking about facts maybe not.
Bitcoin has a limited supply and advantages that can provide added value because of the issue of maintaining value so to compare bitcoin with fiat currencies in a certain category is very different.
If volatility is a measure then the change in fiat currency is not so fast for price issues and more simply volatility is a measure of the movement of an asset that moves quickly for opposite or vice versa issues.
For example, if bitcoin moves so fast in the process of rising or falling prices in the market, it is different from the journey of fiat currency itself so that both are different in the context of value, not in terms of price.