Reading the article, it looks like they did test on Mac hardware, just doing the initial distributed test on cheaper hardware first and then confirming each one individually on more expensive mac hardware. Or at least that's how I read OP.

Yes, I think that's right. I meant they could run the distributed test on the Mac cloud.

It'd be way more expensive, they might as well just fuzz with WebKitGTK+ since it produces the results they desire; then validate the crashes on Safari.

Right but it'd be a lot more expensive, no?