Pretty sure the burden is on you to prove it is not just a method, but also the only method.

Most trivial things can be measured outside time. Many jobs measured on time, we already know why they are measured on time (low activity, presence-based jobs). Anything else, measuring based on behavior/time spent is management being lazy and not doing what they should be doing: comparing inputs and outputs, and measuring the impact of changes to them to improve productivity.

If a manager can't do something that basic, they shouldn't be looking for more bad proxies to hound people with. They should be doing the company a favor and quit.