the text says that the major and minor 3rds generated in the Pythagorean scale are not very optimal - "the minor 3rds are bitter and unsatisfying, its major 3rds harsh..."
In my opinion, a minor triad with a Pythagorean third of 32:27 is perfectly acceptable even though it is definitely different from a minor triad with a third of 6:5. I don't perceive a strong functional difference, and I certainly don't find the Pythagorean interval "bitter and unsatisfying."
By contrast, I do find the 81:64 major third harsh and the 5:4 major third restful and pleasant. Still, the 81:64 third may be acceptable or even useful in some contexts, notably in the dominant triad, which may benefit from the tension of the "harsh" third increasing the Listener's desire for resolution to the tonic. The tonic, by contrast, should be more settled, so my preference for a 5:4 major third there is stronger. This is why I mentioned the relative lack of "functional difference" with the minor triads.
I suppose that this difference is because of the relative complexity of minor triads with respect to the overtone series. The pitches of a 4:5:6 major triad are all overtones of an implied pitch two octaves below the root, and the three difference tones are 1, 1, and 2, two of which are this implicit bass pitch and the other is its first overtone. With a 64:81:96 Pythagorean major triad, the implied bass tone is a full six octaves below the root, and the difference tones are 15, 17, and 32: instead of having two difference tones reinforcing each other two octaves below the root, one is roughly a half step lower than that and the other roughly a half step higher. With a just minor triad of 10:12:15, the implied common bass is three octaves plus a major third below the root, which isn't a chord tone, and the difference tones, 2, 3, and 5, define a different (but closely related) triad, the major triad a major third below, the ♭VI, if you will. For a Pythagorean minor triad of 54:64:81, the implied fundamental is five octaves and a major sixth below the root, and the difference tones are 10, 17, and 27, or, in the C minor example, a rather flat G1, a somewhat sharp E♮2, and C3. Instead of a well tuned ♭VI, they define a poorly tuned major triad on the same root.
In other words, the difference between the just and Pythagorean major triads is the difference between perfect acoustical alignment and poor acoustical alignment, or between high consonance and moderate dissonance, while the difference between the minor triads is the difference between fair acoustical alignment and poor acoustical alignment, or between moderate consonance and moderate dissonance.
This left me wondering whether this is an optimal major and minor 3rd? Ones that are the most pleasing to the ear? If so, how would they be defined?
Not only is this subjective, it also depends on the harmonic context. As noted, the Pythagorean major third might be more acceptable in a dominant chord than a tonic chord; you may find a similar preference for one tuning or the other of minor triads depending on the context, too.
"Pleasing to the ear" does not necessarily mean the same thing when listening to a single sonority in isolation as when listening to a complete piece of music. Discussions of tuning often equate consonance and "pleasing to the ear." But even if we eliminate the subjective judgement about individual sounds by calculating or measuring actual acoustical phenomena, we're still left with context: a major seventh or a minor second is objectively "harsher" acoustically, whatever its tuning, than a perfect fifth, but it may be a most beautiful sound nonetheless.