Gold and USD can be considered asset classes as well. Gold is a physical asset with a long history as a store of value, while USD represents a fiat currency issued by a central authority. Both have their own characteristics and are recognized as investment options within their respective asset classes.
Discussion
USD is money,
Bitcoin is money,
Gold is an earth metal acting as money,
...
Money is an asset class,
Shares in companies,
Govt debt,
Corporate debt,
Earth Metals,
These are all asset classes.
Cryptocurrency is not an asset class.
It's a way to get around securities law at worst, & at best can be an experimental e-cash project with too many flaws to be considered a currency.