It's the consequence of the way chords are named.
There are major and minor chords in the same way as there are major and minor keys. In the key of C minor the root chord is the chord of C minor. C Eb G C. In C major it is C E G C.
Normally, the major chord is just written C, whereas the minor chord is written Cm.
If I add a 7th to the chord of C major, I have 2 choices, either the normal (leading note) 7th, which is a B; or the flattened 7th which is a Bb.
In music theory, the B forms a major 7th and the Bb forms what is called a "minor" 7th. Both these are in the key of C major.
Minor 7th here refers to the interval between the tonic C and the 7th. It doesn't mean that the chord is C minor. This is where it gets confusing.
So C E G B is C major, with added major 7th, to give its full title
C E G Bb is C major, with added minor 7th
The 1st usually gets shortened to C7 and the second usually to Cmaj7
Both of these short forms leave out a lot of information and are used because it's convenient and we all understand what they mean.
If you are in the key of C minor it gets even more complicated
If you add a 7th you can get
C Eb G B which is C minor with added major 7th **
C Eb G Bb which is C minor with added minor 7th
You can see the potential for confusion now!
If I say "C minor 7th" do I mean C minor with a major 7th or C minor with a minor 7th or C major with a minor 7th?
The answer is that this naming system for chords is not an exact science and has been simplified to make it easier to use.
There are conventions which guitarists follow where we all usually agree what, for example, C7 actually means.
The use of tabs makes things clearer. Then we know exactly what chord is intended.
To answer your question, the author is sort of right. It is what is usually meant by people when they use the expression.
A "minor" 7th can, strictly speaking, refer to the fact that the chord is minor, and also that the 7th is flattened.
To get around this, the term dominant 7th is used to indicate the case where the chord is major and the 7th is flattened.
As I said, in a major key, it's only the 7th chord on the 5th note of the scale (the dominant) that is naturally major with a flattened 7th.
This is why major chords with a flattened (=minor) 7th have come to be called dominant 7ths.
However, the 7th in this chord, because it is flattened, is called a minor 7th. This distinguishes it from the case where the 7th is not flattened, and is called a major 7th.
So, 7ths can be major or minor, and chords can be major or minor.
The author is, I assume, attempting to simplify this. When he says minor 7th, he is referring to a minor chord with a 7th. When he means major chord with a seventh, he uses the term dominant 7th.
It is confusing. Guitar chord names, in order to be useable, don't tell the whole story; and that's where the confusion lies.
I spent years studying music theory (piano) before I picked up a guitar. I found the naming of guitar chords very confusing initially because of just this way of simplifying the names of the chords.
I hope I've explained this reasonably well, but I may well have just made it more confusing!
** C minor with an added major 7th [C Eb G B] or any similar chord in another key sounds very strange. I can't say I know of any song which has this chord.