But in this case, it's subtly different.

This issue relies more on a quirk of how PDF and PostScript relate (PDF is built on a subset of postscript).

Imagine you had an image format which was just C which when compiled and ran produced the width, height, and then stream of RGB values to form an image. And you formalised this such that it had to have a specific structure so that if someone wanted to, they didn't have to write a C compiler, they could just pull out the key bits from this file which looks like ordinary C and produce the same result.

Now imagine that your website supports uploading such image files, and you need to render them to produce a thumbnail, but instead of using a minimal implementation of the standard which doesn't need to compile the code, you go ahead and just run gcc on it and run the output.

That's kind of more or less what happened here.

It's worth noting here that it's not really common knowledge that PDF is basically just a subset of postscript. So it's actually a bit less surprising that these guys fell for this, as it's as if C had become some weird language nobody talks about, and GCC became known as "that tool to wrangle that image format" rather than a general purpose C compiler.

The attackers in this case relied on some ghostscript exploits, that's true, but if you never ran the resulting C-image-format binaries, you could still get pwned through GCC exploits.