The idea that this is a good guide for consumers to find responsive retailers has two hurdles. First, retailers who know they have issues would simply opt to not participate, which itself skews the results, along with the fact that many merchants (especially smaller ones) are unaware of it. Second, there is little to no verification of the results. Beyond delivering misleading or outright wrong figures for customers, it does a disservice to honest retailers. Without verification and a much more representative sampling of retailers, it's not clear how meaningful these declarations—whether or not they are on Google ads—can be.
Google is trying to expand its Google Trusted Stores program—where participating retailers give Google average shipping times and Google grades them on customer resolution issues—by letting merchants display the results in Google ads. But there's an issue with the Trusted Stores program itself, and it speaks to trust.