Thanks again - this was a bit of a surprise!
If I may submit an extremely pedantic music nerd bug report: at 46s in the video demo (https://www.youtube.com/watch?v=qboig3a0YS0&t=46s), the display should read Bb instead of A#, as the key of C minor is written with flats :)
(The precise rule is that a diatonic scale must use each letter name for exactly one note, e.g. you can't have both G and G# in the same key, and you can't skip B. This has many important properties that make music easier to read and reason about, such as allowing written music to specify "all the E's, A's, and B's are flat" once at the start of the piece instead of having to clutter the page with redundant sharps or flats everywhere.)
Using exclusively sharps (or flats, but that's not so common) is for piano technicians, frequency-to-note calculators, and similar utilitarian situations that aren't in a diatonic context.
Aside: this is also an easy way to explain double sharps and double flats. If you stumble upon one, and decide to see what would happen if you eliminate it in favor of an enharmonic equivalent (i.e., a natural), you'd end up with a scale that uses some letter twice and also skips a letter. The double sharp/flat achieves the use of each letter exactly once. A bit cumbersome on most instruments (keyed instruments especially), but it does make for easier sight reading (vocals especially) when stepwise movement uses each line/space of the staff, rather than skipping.