I often hear that improvements of mining technology (more hashes per kWh) would improve Bitcoin energy usage. Isn't that just a misunderstanding?
Yes, it is!
Let's say there were miracle ASICs invented that have 1000 times the hash rate per kWh as current ASICs. What I think would happen is this. At first, miners changing to the miracle ASIC would be hugely more profitable than the others. So others would change to the miracle ASIC, too, and the global hash rate increases, and thus the difficulty. The miners using the old ASICs would get unprofitable and have to scramble to get the new ASICs, too. In the end all would change to the miracle ASICs, some would give up, some would come new, the difficulty would be 1000 times than before and the energy usage just about the same.
Perfectly described and it's not an experiment it's just what has happened for years!
There is only one thing that decides the amount of energy spent, and that is the daily reward! If you have $10 as a reward miners will not be able to spend $100 to mine, if the reward is in the billions then prepare for them to have billions to spend on energy!
Am I missing something important there?
The only thing you're missing is part of the reason for this misconception.
People see the hashrate rising despite the price going down and think technology is keeping energy usage lower while increasing the hash rate, but in reality, is just the lag between the price and the deployment of ASICs! So in a scenario where the prices rise to 300k for a week, you won't see the hashrate matching the price, but even if it falls down to $100k you can for sure see both the hashrate and the electricity usage slowly rise to three times what is now.