The comment that confused you is largely wrong and confusing, so don't fret about it.
But the 5+5=9 part is correct. It's a weird artifact of note-counting. C D E F G is 5 letters. C to G is called "a fifth" (which is a horrendous use of language, but it's a perversion of saying "we got to the fifth letter"). G to D is also "a fifth". Stacking them means C to G and G to D, and you don't count the G twice, that's why 5+5=9 instead of 5+5=10. But if we had actually counted the steps through the letters instead of counting the starting note, it would be 4+4=8, and that's logical and correct, but you can't say that to musicians because that's not how music jargon works.
Yes, ratio math works without the strangeness. Except it's unfortunate that the 3rd harmonic is the fifth letter and the 5th harmonic is the third letter. Coincidentally, from harmonics 7 through 14, the harmonic numbers match the letter-count numbers. That's partly because 7:8 is the beginning of the harmonics being roughly whole-step sized, and 14:15 is where they shift into half-step size.
How does 5+5=9? (Wrong I know but what is right?)
Isn't the 9th harmonic a distinct frequency in a musical note? I thought it was side effect of how physical vibrations ripple out or something...
Where can I learn more about what you're talking about? It's very interesting. Thanks.