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)."
[1] https://grsecurity.net/respectre_announce.php