We got our 2nd edition of The New Grove last week, shipped end of the week before IIRC. That is what I was quoting.
The problem with a dictionary vs. some music theory text is the underlying rules. Most dictionaries are descriptive. If a word is used a particular way enough, well then that's what it means. The thing is that meanings change. The word chord has been around for "sounds in agreement" since at least 1475 (OED)
OED gives 1752 as the date for it meaning either concordant or discordant & three notes & that's in a music theory treatise. It also give an example of: 1875 Theory of Sound vi. (1883) The above chord is the most consonant that exists in music, and it is therefore called the perfect chord. Can a 3 note chord ever be defined as perfect? I thought all the perfect sounds were 2 note combinations.
I'm inclined to believe music theorists co-opted a generally known & understood term and redefined it for their own purposes. They also needed easy shorthand for designating complex sounds so you get major, minor, diminished, augmented, inverted & so forth. Maybe 2 note chords are easy enough to indicate that they didn't need to develop a shorthand way to talk about them.
I see no reason why the inability to label it in some kind of shorthand means it can't be a chord. That's just backward thinking.