I looked at that. If you assume that people in different areas behave in the same way with respect to social contact, even if they behave in different ways wrt. vaccination, then it supports your conclusion. The study tries to average that out, but...

There are studies that don't have this weakness. For example, you may look at households, on the assumption that people will have so much social contact with their households that the differences don't matter. Here's one that looks at that. "We estimated vaccine effectiveness against onward transmission by comparing secondary attack rates among household members between vaccinated and unvaccinated index cases, based on source and contact tracing data collected when Delta variant was dominant." https://www.medrxiv.org/content/10.1101/2021.10.14.21264959v... I've seen another study with comparable results (the error bars overlapped).