it has no real meaning. Why would you want to sample a 10kHz square wave anyhow? After all, it contains the fundamental and all the odd harmonics, 30kHz being the first odd harmonic. Silly exercise, really.

What they should have shown is music being sampled at the different rates and see that, not the sq wave.

What you really need to do is get the Cd "The Digital Audio" by Marcus Erne, available at Amazon and do some DBT comparisons of fully sampled music and the same band limited at different frequencies, like 5.5kHz, 11kHz and 16kHz. Or at different bit rates. Yes, now that is an eye opener, not sampling the square wave. It also has some other real eye openers on it. Have fun.