I don't think it's a joke, for the general case of print this is indeed a problem as the whole print industry runs on CMYK. You could go further and say that because CSS/Weasyprint doesn't have a Pantone plugin, it'll be difficult to control colour accuracy throughout the production process. You're right this isn't necessary for most novels, but it's a very real concern for print media.

It is not a joke. Color print exists, you know. For example, try obtaining a pure cyan from RGB color...