I'd say it's actually easy to evaluate a leader, at least one with experience under their belt. You look at the performance of their organization. If they're a CEO, it's especially trivial: you look at the performance of the entire company. If they're a VP or Director of a smaller section of the org, you can still measure that section's performance, though it does get a little muddier.

At a decent-sized org it is nearly impossible to measure an individual developer's impact with any granularity.

I think this should also illustrate that evaluating a leader and evaluating a developer are two entirely different things that can't be directly compared anyway.