I don't think that's the only way they flag VPNs. If that was the case, student accommodations, hotels, offices and the like would be flagged pretty quickly. I

I don't think that's the only way they flag VPNs. If that was the case, student accommodations, hotels, offices and the like would be flagged pretty quickly. I do believe one of the things they check is if the IP address belongs to a datacenter range (e.g. AWS, Azure, GCE, OVH, etc) or to an ISP providing Internet access. It is just the opposite of what some anti-spam RBLs do :)