I agree that asymptomatic transmission reduces this pressure, but symptomatic SARS-CoV-2 carriers are often still contagious, and there is more public will to enact extreme precautions for more lethal illnesses.
So there is still some evolutionary benefit for lower lethality.
A variant of COVID as lethal as MERS would spread just as well since carriers are asymptomatic most of the time they are infecting others.