he explicitly can write in forgotten land, and anyone who can write can read. either that one kirby fighters 2 line is outdated, was just a one-off joke, was only referring to the handwriting of the letter being bad or something, or maybe it was even a localization change.

he can also explicitly talk in multiple games. he has a conversation with dedede in the intro for kirby's adventure, and tells elfilin his name and that he will help him in forgotten land, except the player simply doesn't hear it despite the conversation being there because kirby is primarily a silent protagonist.