I'll concentrate on the historical part of your question as others are doing well on the modern work. As you've suspected from the etymology, the term δίφθογγος is Ancient Greek (diphthongos, borrowed into LL as diphthongus) and was already in broad use by the 2nd century CE - e.g., Terentianus writes:
Porro vocalem secuta vim tenet vocalium
Et sonos utrosque jungit, unde diphthongos eas
Græciæ dicunt magistri, quod duæ junctæ simul
Syllabam sonant in unam
Moreover the following vowel has the force of a vowel
And joins both sounds; whence the diphthong,
The Greek masters say: the two joined together
Sound as one syllable.
...and just as old are arguments about it. Is the diphthong two, or is it one? Is it a single sound regarded as a unit, or two sounds yoked together? Pennington's 1844 An Essay on the Pronunciation of the Greek Language reveals this term has provoked long disagreement since classical times.
Now, because there has been a single term for the diphthong, we can say that it has been regarded as a single entity (of some type) since classical times, though its nature has been disputed. The ancients did know that a diphthong was sounded in a single syllable; they typically regarded as the syllable as the smallest unit of sound. So I think that our own usage today of diphthong as being in some sense a 'single sound' is a relic (correct or incorrect, as others discuss) of that time.