That number sounds way high
What ever the number is it needs to be more.
According to this article there are around 52 million "every day" millionaires in the US worth between $1-5M. It should be trivially easy for these people to own and self custody one bitcoin (except of course the obvious 21M BTC reason).
We need more easy to use self custody wallets like Unchained, Casa, AnchorWatch, Proton, Bitkey, Liana, Sparrow, etc. Bring on all the projects with what ever models/trade-offs work to somehow put the keys in users hands.
https://www.cnbc.com/2025/06/19/united-states-millionaires-wealth.html
Discussion
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