Nobody said a 20A breaker can't be used. But not with 12awg wire derated that much. If the circuits can use 15A breakers then the wire is fine. If the circuits require 20A breakers then they need larger wire or an arrangement with less of a derating factor.
Lets use Dsg319' example there are 18 CCC for heat traces at 15 amps per heat trace
that means 15x125% = 18.75A wire
to derate the wire it's 50% meaning you need 37.5A wires
that means you need a 20 amp breaker and a minimum 37.5A wire for each heat trace correct?