Definition of Asset Class from
Investopedia:
An asset class is a grouping of investments that exhibit similar characteristics and are subject to the same laws and regulations. Asset classes are made up of instruments which often behave similarly to one another in the marketplace. Historically, the three main asset classes have been equities (stocks), fixed income (bonds), and cash equivalent or money market instruments. Currently, most investment professionals include real estate, commodities, futures, other financial derivatives, and even cryptocurrencies to the asset class mix.
Goldman Sachs might personally not like Bitcoin, they might be right that Bitcoin doesn't have all those properties that they like to see in investments, but there's absolutely no reason to say that it's not an asset class. Currency is an asset class too, anything is an asset class if it can be consistently used for investments.