Thanks for the feedback! I strongly agree this is not the only measure of developer productivity -- but it's certainly one of them. I think this measure as speaks very directly to how _many_ developers (myself included) understand the impact of AI tools on their own work currently (e.g. just speeding up implementation speed).
(The SPACE [1] framework is a pretty overview of considerations here; I agree with a lot of it, although I'll note that METR [2] has different motivations for studying developer productivity than Microsoft does.)