Mariner 9 said
...the fact is that speed is a function of both attitude and power, so can be controlled by either throttle or elevator in isolation...
That statement is absolutely and fundamentally incorrect. Speed is controlled by a combination of power and elevator. The whole point is that you can't control it by one without the other (in level flight, or without changing the descent or climb profile you are using).
Thinking about point-and-power as a different technique that "proves" that power controls speed is not really helpful. Either way round the adjustment to speed and/or approach angle has to be done by a combination of adjustments to both power and pitch.