By your logic, one note should be chord. Why two?
Let's not worry about the word 'chord' any more - You're happy using it your way, I'm happy using it my way (And I'm happy with you using it your way, and as I said from the off, I know many others do too...)
But the big difference between one note and two is...
With one note, there is a succession of harmonic partials that are all multiples of a fundamental frequency. The ear is designed to recognise this relationship as something that happens when 'a thing' is vibrating at its various modes of resonance, and hears it as 'a sound'. (I know that this is a gross simplification, and not true of all instruments, but it is a close enough description of how most western instruments work).
With two notes, the full set of harmonic partials that the ear hears is no longer based on a single fundamental. So, normally, the ear can "do the math", work out which partials belong to which 'cause', and hear two notes.
And yet, if those notes are related by a simple enough frequency ratio, as chosen by a composer (or a 'scale designer'), there will be a number of sequences of evenly spaced partials within the overall set. So if we have a note at 1000 Hz and one at 1500 Hz, then the ear can pick those out. But hey, we also have many multiples of 750, and 500, that are strong as they're present in both our original sequences. What's the inner ear to tell the brain? Thankfully, it hedges its bets, and we get the beautiful sensation not just of two separate notes, or 3 or four, but of harmony.
3 notes extends the effect further, as does 4, as does 5 - but the jump from 'a sound' to harmony is made as soon as you introduce that second note.
So for me, 2 is the magic number. Which doesn't mean I don't appreciate more... I've been listening to Steely Dan all week.
We are hijacking someone's thread.
Kinda, but if we keep going I think we might meet in the middle somewhere...