That's true, but that still doesn't explain how the chord
functions. What makes a "cadential six four" chord cadential? Or more practically-- which second inversion triads are cadential, and which aren't?
To be completist about my criticism here-- the author could stick only to figured bass analyses and give enough examples that the reader/listener starts to implicitly hear the bass patterns, phrase structure, and voice leading that help them differentiate between passing 6-4 chords and cadential ones. Eventually, we've sung enough chorales that we can quickly start parsing these patterns easily enough.
However, the author chose to introduce Roman numeral analysis which is the explicit approach to demarcating how the chords in an example function harmonically. That explicitness is quite useful. E.g.
* the piece begins to modulate using this specific chord as a pivot, specified with roman numerals for both the old and new key
* this second inversion chord gets special notation because it's a cadential six-four which will soon resolve to a V in root position
That would all be fine and appropriate, except that the author then relegates cadential six four chords to an ancillary part of the tutorial. But demarcating cadential six four chords is one of the big reasons to go the route of Roman numeral analysis in the first place! Teaching students how to explicitly spot and notate the difference between passing moments in the phrase and cadential moments for the same exact chord is key to them understanding how phrases of tonal music operate.
In a normal harmony class, the lessons dealing with cadential six four chords make it easier for students to more quickly analyze difficult music. E.g., is the first chord of Wagner's Liebestod a "one six four" chord or a cadential six four? In Roman numeral analysis one must resolve that ambiguity by making a choice. When one does, the structure of that part of the opera becomes obvious-- it's an ending full of little endings, each of which get elided through chromatic voice leading.
You can eventually come to the same conclusion using only figured bass. But chromatic 19th century music can get pretty hairy and so the explicit tool of Roman numeral analysis can come in handy there.
But if you learn Roman numeral analysis without learning about the quite common cadential six four, it strips out one of the fundamental lessons for figuring out how phrases of tonal music build and release tension in time. Consequently, analyses like this become more difficult for no good reason.