Seems like a No True Scotsman fallacy to say only good technical leaders can tell if a developer is being productive and in the same breath say it's unmeasurable.

hours worked, bugs fixed, tickets closed, costs saved, clients saved, KPIs/OKRs hit, time in queue, hours-to-close-ticket, uptime, SLAs hit... surely some collection of indicators, while not a pure signal, would let you highlight outliers either above or below the curve.

It's like saying, "your productivity as a carpenter is how many houses you build in a year" while refusing to take into account how big or complex to build are those houses.

Even "hours worked". What do you mean, hours spent in the office? How do you know the person was not drinking coffee or staring at their code in thought of something else.

That being said, I think your metrics are good. And even single developers should be measured by them (which means all developers get measured by the same metric and get the same value). Why? Because it helps the business of SLAs are hit, no matter why they are hit.