This sounds like you're either forced to work with them, or they can just publish pretty much anything and claim plausible deniability about whether it's true or not?

I guess but what's the alternative? Either leave the info out or make maps paid to cover the cost of human reviewers.

The alternative is just only including info from registered businesses. If a third party provides info on an unregistered business, Google could reach out to the business to confirm and ask if they want to register. Unless they say yes, they can just not include it. Having a smaller set of correct information is better than a larger set of mixed correct and incorrect information is better for pretty much everyone...other than Google, of course.

I am a Local Guide and Google never prompts me to confirm anything.

Local Guides are ordinary unpaid Google accounts who submit reviews, photos, and other edits as I’ve detailed here. We are sometimes prompted to answer questions, but only with a blank to fill in.

https://support.google.com/maps/answer/7084895?hl=en

Google says "We review all the edits you make."

Go maps app > contribute > answer yes / no questions to help others