... which someone here said is how 400 became the end of Zone I or level-off height.
No, that 400 feet is a holdover from CAR4B and the pistion airliner era.
Also consider screen height, jets 35 feet, piston aircraft, 50 feet.
Why the lower figure for jets?
Believe it or not, some of the early types (B707-120 for example), simply could not meet the fifty foot requirement anticipated, performance-wise, so the screen height was lowered to 35 feet.
Some of those early models were...lead sleds, in the extreme.