You're right, digging in a bit most of these "every day" millionaire likely have it all in illiquid assets, mainly their houses. But these people do have the option to downsize a little and not use their primary residences as their only long term savings accounts.

A related article from last year says there are around 5.5M people in the US with more than $1M in liquid, readily investable assets. I doubt many of them own, let alone self custody any bitcoin.

https://www.cnbc.com/2024/03/22/the-us-is-the-top-country-for-millionaires-and-billionaires.html