I also found another hint about their findings in this PDF written by Yarden's co-researcher Alex Ionescu: https://www.usenix.org/system/files/woot20_slides_ionescu.pd.... One of the slides specifically mentions the use of fuzzing tools to find these issues.
If there are other, better links I don't know about, please kindly share. :)
Here's a tweet from the original finder: https://twitter.com/gabe_k/status/1330966182543777792?s=20
Yarden & Ionescu's work are both really top notch. Also anything by Google Project Zero if you want to do a deep dive on the subject.