This may be hairsplitting, but emphasis on the handling word in that sentence. I would bet it's not so much the feature gap it's that the specification is very, very YOLO. And when any number of byte sequences happened to render as a PDF[1], then such a situation leads to a very, very diverse set of inputs that coincidentally work on some systems leading to "works on my machine" type outcomes. When your project is in the business of rendering things which happen to work in ${other system} it leads to a lot of very angry users

1: it's like the million monkeys with a million typewriters decided to go work at Adobe and they really have heard great things about mixed text and binary file formats because they are outstanding RCE and stack smashing opportunities