Seconding this as another Google engineer (I work on data quality for Google's Knowledge Graph). There would have to be
at least four utter failures and drastic-divergences-from-past-behavior spread across multiple organizations, multiple bureaucratic and technical processes, and potentially even multiple
companies.
My experience has been that Google's internal culture has always been extremely serious about restricting access to privacy-relevant data. On top of that, there's recently been a big push to defend against malicious internal actors, things like engineers intentionally creating backdoors or the like. You'd have to work to specifically and intentionally override defaults to mislabel the data and every piece of code that works with it, actively defeat multiple layers of access-control tech, get the code to accomplish that past privacy-and-security review by engineers from the privacy and security team, lie continuously and fake a bunch of supporting evidence when requesting about four different kinds of quota, hide your column names and API definitions from infrastructure engineers doing migrations and routine load-management stuff, lie to a bunch of lawyers and general Search+Ads PMs during launch reviews, and more. It's just not going to happen.
It's also not going to happen because "someone at the top told everyone to do it and nobody complained", either. Googlers pitch shitfits like no employee body I have ever seen, and using health data to drive ads would instantly cause internal messaging to explode into a tornado of hatred. I've seen it happen for less.
Like QuercusMax said, using totally-aboveboard anonymized-and-aggregated-and-scrubbed query logs is already hard enough, enough so that my team has a policy of just plain not doing it. I have a coworker whose account is irrevocably tainted because he used to work on a project that used data from the "This result is wrong" button on search results with biographical information about public figures. Misusing health data like this would be so obnoxious and difficult as to be unbelievable.