They're measuring how much work they're getting done based around approximate predicted complexity / work amount using story points and then using that to map overall work output and charting it from between non-WFH and WFH. If the projects they're working on have tended to remain of similar complexity with people that tend to be consistent, then this seems like a perfectly fine metric for exactly what they're trying to discuss: the relative output from WFH and non-WFH.