What I called complexity is an amount of different nodes (not sure this is the proper term) in a query plan. So, the more complexity for a given cost - the more work JIT needs to do.

But the planner sums each step in the plan so isn’t a more complex plan already more expensive? Or is the addition of another step not adequately considered in that cost?

A more complex plan isn't already more expensive in a way that captures how much work the JIT will need to do. I think I'll need to write a separate blog post about that, with details and examples.