Throttle couple advance is not the best way to control spark timing on an engine, but it's good enough on a model plane motor. The reason the old cars had vacuum advance as well as mechanical advance (rpm advance), was because when the vacuum is high, the engine load is low, and power and economy improve with more advance. The model plane electronic ignitions have an advance curve based on rpm (like the centrifugal mechanical advance in the old cars). For steady state running (not accelerating or decelerating) at a fixed rpm, the engine load doesn't change a lot while flying. It changes a little with flying conditions and if you change the prop. If you added a load sensor (MAP sensor or vacuum sensor) to an electronic ignition, all it would do would be to decrease the advance on acceleration when you slammed the throttle open, and gradually the advance would be added back as the vacuum increased.

The throttle coupled advance is only related to rpm under steady state conditions. On acceleration, it gives more advance than is required. If you slam the throttle to full open, the ignition advances fully before the engine can reach max steady state rpm.

Either of these systems provides a timing curve that is closest to optimum if you roll the throttle on slowly, so the engine rpm can catch up with the throttle opening (and steady state vacuum).

Next we can talk about the lack of accelerator pumps on the diaphragm carbs, and how the mixture has to go lean on fast acceleration unless the carb is set richer than required for steady state running.....