Indicated airspeed, as sibling comments point out, is not true airspeed. The air is moving at 500 but the stagnation pressure is the same as if it were moving only 300 at sea level so the indicated speed will be 300 +- calibration errors.

My takeaway/learning is that "knots indicated airspeed" is a measure of pressure due to the aircraft mushing its way through the atmosphere, and is vaguely in proportion to actual_airspeed * altitude, and from my non-pilot's perspective ought to be expressed in bars or torr or something else meant for pressure and not velocity or distance.

The airplane flies based on how much of that mush is doing work on various parts of it, which means "I stay in the air" is a complex calculation of speed and pressure. Factoring in the slowly-decreasing density of that mush to those very important speeds saves calculation time, which is why pilots stick with indicated as the baseline. It's a known-wrong measurement with some valuable uses, and true airspeed and ground speed are usually available in the cockpit in real time. They're just not flown by.

IAS is calculated from the difference between the outside air pressure as measured by a "static" port on the side of the aircraft and the pressure measured by the pitot tube pointed in (hopefully) the direction of travel. It could be measured as pressure, but it is proportional to true airspeed under standard temperatures and pressures.

The short answer is that bus left about a hundred years ago.

See about page 10-24:

https://www.faa.gov/regulations_policies/handbooks_manuals/a...

You are not going to like 'density altitude' then, and the terms 'light year' 'parsec' and 'electron volt' will blow your mind.

Despite the efforts of standards bodies to regularize everything, people continue to use measures that match their purpose.