Obviously, he knows how the market moves that's why he made the decision in 2021. He knows that within the next 2-3 years, Bitcoin will be on a accumulation phase and since his salary is pegged to USD, he already has a significant profit because of how much Bitcoin he got on the past 2-3 years. 
It's belief, not about knowing anything for sure. Bitcoin is good and has too good ROI record but it does not mean you know well that how it will be in 2025 when you were in 2021 year.Quote
Forget the critics, and believe in yourself. If you know more than those who are criticizing you, who are them to follow what they're saying to them. The problem with fluctuations is that, if they give the salary today for example and a few days after that, the market dropped. That could be a problem, but aside from that there's no problem for it. It's just that, sometimes people are overexaggerating things especially when it comes to this kind of thing where there are people who wants to have their salary in Bitcoin.
Believe in Bitcoin but before that, doing your research and learn enough about Bitcoin fundamentals and don't have like blind belief without knowledge. You can blindly belief in Bitcoin and get profit but if you maintain it with altcoins or any other things, it's high risk that you will lose money.So practicing it with due diligent research on anything before investing in it is very good practice and it can build up a habit for safe investment decision making and even in other things beyond invesment.
Market drops are chances for accumulation with discount prices and likely only knowledgeable and experienced people in this market can do this.