You seem to be missing the point. An octave is a difference of seven scale steps, just like a second is a difference of one scale step, and a unison (a "first") is a difference of no scale steps.
The fact that this interval of seven notes is called an "octave", named after the number 8, is entirely because of our 1-based system that double-counts the note you start on.
If we had a 0-based system, it wouldn't be called an "octave". It would be named after the number 7. Let's call it a 7-interval, to avoid confusion with actual music terms.
In a 0-based system, you could say, for example, that two 2-intervals make a 4-interval because 2 * 2 = 4.
In the 1-based system we really have, you have to say "two thirds make a fifth", and the mathematical connection is not obvious.