Allowing Google to run an ad campaign targeting their members wasn’t the benefit Blue Cross was talking about, that’s a side effect from them not turning off the data sharing option in the Google analytics settings.
The analytics data is used for prioritizing development work. If a tool they have on the website relies on a library that isn’t compatible with a new version of React, for instance, do they know how many people use it? Having analytics allows you to decide what’s worth spending the development time to maintain.
I think mistakes like this are usually caused by someone changing their mind on one thing they wrote and forgetting to proofread the whole thing to see if it still makes sense. I imagine this sentence started out as “Rock the size of a small boulder”.