https://www.ncsl.org/research/health/state-quarantine-and-isolation-statutes.aspx
Quarantine laws have been on the books, in every state, for hundreds of years.
We just haven’t had to use them in several generations.
Quarantine laws, implemented by a legal authority are valid. A bunch of guys from the local pool are hardly a legal authority.
Actually, under the Constitution, the police power was reserved to the states. Quarantines would very likely have been routinely accepted by the founders if imposed by state governments.