I'm not sure to what degree EdX has really "solved" this. My impression is there's quite of range of quality and rigor on EdX as well. And, more centrally, most people still don't see EdX certificates as general substitutes for more conventional educational degrees.
The EdX solution to this is that their courses are all endorsed or run by brick-and-mortar universities with considerable investments in their brand that they won't want to tarnish by attaching it to any random certificate.