If I understood what little I read of the exploit details, it actually loads an opaque payload through WebGL calls, so simply crawling with an iPhone's user-age

If I understood what little I read of the exploit details, it actually loads an opaque payload through WebGL calls, so simply crawling with an iPhone's user-agent wouldn't be enough to notice anything "fishy". The site can just serve the payload to all visitors, then the implant will get installed if the resulting kernel calls are broken as described in the article.