Come on, since when was King ever cowardly? Blocking and avoiding attacks that are capable of damaing you doesn't make anyone cowardly, because otherwise Kaido, Luffy and Zoro are also cowardly.
Plus, if he fears pain, he wouldn't be blowing himself up after willingly getting stabbed.
The only narrative reason to have that part, is to convey that things have changed since before. Earlier, King can allow Zoro free hits and laugh. But now, they deal damage.
And you yourself say, King doesn't know it for sure. King thinks it will damage him. Who knows King's limitations and lunarian trait better than King himself?