Oh, well
..."for years before 2018 estimates [of percentages held by Americans] relied on....search trends,
bitcoin forum participation..."

...including, I assume, public guesstimates of BCT member stashes.

Seriously, though, I think that this almost 50 mil number is VASTLY overestimated if IBIT/FBTC/MSTR is not counted.
Besides - what is the meaning of gold ownership? Wedding rings and earrings and necklaces don't count? I mean if you had a wedding rings worth of bitcoin you would certainly qualify as a bitcoin owner.