OK, but there is no way that the reversers alone could achieve the 2.4 m/s/s required for the MED setting
I agree. But there was also no way for the crew to know if the brakes/wheels alone had sufficient grip to achieve the required deceleration on that wet runway.
I agree that reversers are a distant second to brakes when it comes to slowing down the aircraft but in wet conditions their importance increases markedly.