A stable coin is a coin that do not fluctuate up and down regardless of the market condition. Bitcoin been bought as a results of investors selling their Tether does not make bitcoin a stable coin. Bitcoin isn't pegged to a particular fiat currency on 1:1 ratio hence cant be a stable coin.
Why a stable coin must pegged to a fiat currency to be a stable coin? neither gold nor USD. I think only the unique one trusted by most of the people have this opportunity.