These millionaires are successful in their investments because they have the right mindset, they invest because they already have enough income for all their life needs
What you are referring to is different I believe, because most Bitcoin millionaires are those who invested in Bitcoin when it was in its initial stage and the price was very low, and they kept holding onto their assets until the time Bitcoin got significant value. So, those who invest right now because they have enough income will barely become millionaires if their investments are not that high.
whereas most who fail because they think they can get rich from investing in Bitcoin because their main needs are not yet met.
The mindset of getting rich from cryptocurrency investment is wrong in the first place because one should only have a specific profit target and shouldn't think that they will magically get multiple folds of their investments through certain cryptocurrencies because that barely happens all the time.
The key to success in the world of Bitcoin is investment, not trading.
That's not entirely true, a person with enough knowledge and experience about the market and trading practices can get success through trading as well.