Project Zero disclosed this to Intel, AMD and ARM on June 1 as stated in the blogpost [1]. I believe an independent researcher working somewhere else (or even at home) would have done the same, as indeed the parallel independent discoveries ultimately did.
[1] https://googleprojectzero.blogspot.com/2018/01/reading-privi...
I think it's also less statistically unlikely than you think. Thousands of security issues are discovered every year, it makes sense that some small number will be discovered in parallel. It looks like a big coincidence because you are focusing on the one parallel discovery, but if you consider it in the broad landscape of all discoveries then it's just statistically expected that there would be at least one parallel discovery.
No tinfoil hat: Because the series of research to date connected enough dots it crossed a threshold and lightbulbs went off all over.
Jann disclosed the issue to Intel in June. The subsequent activity around the KAISER patch was the reason we started investigating this issue.
Still, as they say in this article, without a tinfoil hat: "There’s no reason someone couldn’t have found this years ago instead of today."
https://en.wikipedia.org/wiki/Meltdown_(security_vulnerabili...