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.