> the integrity to not sell something like this on the black market

Wouldn't the investigation lead to him? He noted he did not know, at the time, that it was an RCE. So he would need to research further (where he might trigger an alarm). He also noted he utilized staging environment, which he had access to due to previous found vulnerabilities (so Google had his personal details).

So, now imagine he found something and he had sold it. If it was used, Google would do a thorough investigation to find if given vulnerability was abused in the past. And they find this guy using it exploratory and nothing else. It's not hard to put 1+1 together after that.

While the idea of finding an exploit and selling it for hundreds of thousands of dollars on black market sounds exciting, it wouldn't be so easy in this case.