The odds that Google had the first team of researchers to trip over the bug is low. But we know that they were the first team to disclose the vulnerability, an

The odds that Google had the first team of researchers to trip over the bug is low. But we know that they were the first team to disclose the vulnerability, and the only reason not to disclose it is if you wanted to exploit it. So the key question really isn't "how likely was someone to find this", but "how likely is it that Project Zero was the first". I think it's hard to estimate odds, but I'd be surprised if it was even as high as 50%; there's too many teams, individuals, freelancers, state actors, etc. actively engaged in looking for this kind of thing.