While they say that Respectre would have caught it, the actual discovery was apparently made this way:
"We independently backported the fix on July 9th 2019 and on noticing the warning, fixed it correctly. When the upstream kernels later backported their bad fix, it created a conflict in our git repo that led to us immediately spotting their flaw (and keeping our existing fixes)."
"We independently backported the fix on July 9th 2019 and on noticing the warning, fixed it correctly. When the upstream kernels later backported their bad fix, it created a conflict in our git repo that led to us immediately spotting their flaw (and keeping our existing fixes)."