There is no doubt that this is a targeted sample of people, obviously those between the ages of 20-40 who understand technology, who somewhat understand what is Bitcoin and why it is worth having it as a long-term investment. Different studies show different results, so we have information that about 5% of Americans own some crypto, but also that 20% of Americans aged between 18-34 years own Bitcoin.

I think all these numbers are relative, and it is impossible to get accurate figures on such a small sample of people in country that has almost 330 million inhabitants. What fascinates me a lot is the fact that the rate of adaptation is almost always measured by something coming from the USA, which would mean that if Bitcoin fails there, it will not succeed anywhere in the world.

In the following links we can see examples of different research on this topic:

https://www.prnewswire.com/news-releases/largest-bitcoin-ownership-survey-reveals-6-2-of-americans-own-bitcoin-while-7-3-are-planning-to-buy-some-300928651.html
https://www.cryptoglobe.com/latest/2019/05/20-of-americans-age-between-18-34-own-bitcoin-recent-survey-reveals/
https://www.cnbc.com/2018/03/16/why-just-8-percent-of-americans-are-invested-in-cryptocurrencies-.html
https://beincrypto.com/1-in-12-americans-own-cryptocurrency-finds-new-market-study/
https://www.rttnews.com/2997766/9-of-americans-own-bitcoin-survey.aspx