Its hinted that King lost his wings somehow, having his power reflected to him may have spurred their growth. I makes no sense since King was trying to open a wound not heal it, but maybe fullcounter reverted the intent of the power.
Its not like the series has been making much sense lately nowadays, lets just enjoy the pretty pictures.
Yeah I came across the wings as well after rereading the chapter, but as you said it makes absolutely no sense for a redirected attack to affect their growth. And why even pick up this issue in the middle of the whole "Are you good or evil, Meliodas?" business lol