Do you have a public source about an embargo period for this one? I wasn't able to find one

Then even in the worst case scenario, they were addressing this issue two days after it was publicly disclosed. So this wasn't a "rush to fix the zero day ASAP" scenario, which makes it harder to justify ignoring errors that started occuring in a small scale rollout.

Considering there were patched libraries at the time of disclosure, those libraries' authors must have been informed ahead of time.

Cloudflare did have early access, and had mitigation in place from the start. The changes that were being rolled out were in response to ongoing attempts to bypass those.

Disclosure: I work at Cloudflare, but not on the WAF