It also helps to be able to understand the tech enough to know if a certain approach is going to impact other initiatives which are further down the roadmap. One of my favourite questions I've been asked by a PM is "Will any of this work become easier to do if we change the order of these work items?"

I would have thought every manager asks such questions either to himself or to the team quite often.

Not in my experience - juggling priorities is usually difficult enough when you're just trying to manage the C-suite's favourite ideas, or feature requests from the highest revenue customer, etc. Factoring in synergistic engineering efforts is out of reach for a lot of PMs that I've worked with.