It seems like satire because the story isn't, even on the surface, about the need for signs in subway stations. It's about a dedicated employee who, performing what many would consider a menial job, displayed care and dedication to his job and in the process revealed himself as something of a design savant.

Saying "but you can print signs for $50" is so wilfully blind to what makes this story charming that it seems like a parody of the literalness and complete lack of romance that one sometimes sees in HN comments.