Perplexed on how single driver speakers can cover such a large Hz range

Speakers don't exactly create different frequencies and then put them together. The single signal is a complex combination created by a signal composed of all the frequencies a microphone picks up. The signal is a complex of hills and valleys one after the other all combined into one ever changing signal.

An interesting oldie from the 70s was the Rectilinear X(ten) 3 way but the mid range ran from 200 Hz to 8 kHz with a 2 " cone tweeter above that and a 10" woofer in a closed box below so an almost full range speaker. And interestingly the crossover was a series one not a conventional parallel design.

But I agree with lots of us that full range has too many compromises in cut off lows and highs and inability to play loud. But I do love the top to bottom integration. You sense there is no crossover.

Just a closing thought. Perhaps the best full range speakers are a few one way planars and electrostatics. The most wonderful being perhaps the Quad 63 which even could image.