"This is known as an Octave and is not considered a new note. An Octave is the same note with a higher pitch."
Why is this so? I guess I'm totally tone deaf since I get no understanding of the sameness of notes separated by one octave by listening to them. In the same vain the good sound/bad sound example that opens the harmony section sound both good to me.
That is, when you pluck a guitar string that's tuned to 400hz, it also vibrates at 800hz, 1600hz, and so on. (Do an image search on "guitar string harmonics" if it's not obvious how that would work.) But we hear such overlapping tones as a single "note", whereas a 400hz tone overlapping with a 600hz tone sounds like a chord.
And yeah, the "bad" example is not really very bad. It's a little bit tense, but there are much worse ratios they could have chosen.
They aren't the same note: if someone plays A3 (fundamental is 220Hz) and then A4 (440Hz) you can easily tell them apart! In contrast, if they play 220Hz and then 221Hz an untrained listener will most likely hear them as the same note.
In some contexts a note and its octave can be thought of as equivalent: if you are playing a chord you can often replace A3 with A4 (and vice-versa) without changing the chord's feel and function. However, that isn't always true. For example, playing G#3 and A3 at the same time will sound more dissonant than playing G#3 and A4 (a minor 2nd is more dissonant than a minor 9th).
But TBH I know next to nothing about music so I could be off.
Most instruments produce harmonic multiples of the base frequency. (Percussion instruments don't.) So if you play a note at 110 Hz, there are also pitches at 220 Hz, 330 Hz, 440 Hz, etc. contained inside it. If you play a note an octave up, at 220 Hz, it contains pitches at 220 Hz, 440 Hz, 660 Hz... which is similar enough that it sounds like "the same note".
(This doesn't explain why 330, 660, 990, ... sounds like a different note, though. Maybe it doesn't to an untrained ear.)
Two notes played in an octave would be quite distinct since you have a clear first fundamental for both notes, and the loudness/decay of the harmonics is slightly different between both notes