The paper tag on the faucet supply line says iron pipe, so I think it must be that, not compression.
But I need a valve on the 1/4" port. I could go from the 1/4" tee outlet, to a 3ft supply line, to a 1/4" needle valve, to a 10ft supply line. But that seems really clunky. I'd rather not have a valve dangling around on a hose under there.