> If I urgently need to meet with someone today and I see they've blocked off the whole afternoon for a 4-hour meeting, it could make a big difference if the 4-hour meeting in question is "Critical presentation to the CEO" or "Weekly HR thing that HR spams everyone with but nobody attends and doesn't matter anyway"

Unless they are your subordinate, so that you are empowered to judge and direct their priorities, it doesn't. Nothing prevents you from sending them a message (or meeting request) acknowledging that you know they have som thing scheduled but you have an urgent need (and explaining the need) and allowing them to make a decision, or Maki g a similar request of their superior if organizationally appropriate. Your urgent need doesn't transform you into their boss.