This explanation is an excuse, and does not pass muster. I remember adjusting spark timing with a timing light flashing on the harmonic balancer and turning the distributor by hand. Cars two decades before that, which I never personally serviced but have seen, had spark adjustment controls on the steering wheel.

The term was always called spark adjustment, never compression. Compression is the thing you worry about when milling the heads or exchanging oscillating components (rods, pistons) due to changing the physical geometry of the combustion chamber.

So yes, adjusting the spark timing does technically adjust how compressed the air/fuel mixture is at the moment of ignition. But the term used to describe it has always been "adjusting spark" or "adjusting spark timing" or "adjusting timing". Nobody ever refers to that as "adjusting compression" because the term compression is used to refer to something else.