Cadential six-four chords illustrate very well the idea of voice-leading elaboration of a single harmonic function, and in turn the important distinction between chord identity and chord function (see Michael Berry's explanation on his blog)
In the case of cadential six-four applied to the dominant, two chords (I 6/4 & V) appear to be involved but from a functional voice-leading perspective, this is no more than an expansion of V involving a double voice-leading movement 6 à 5, 4 à 3 (see Ex. 7.7 & 7.8 for two simple examples). In other words, the second-inversion tonic chord is part of a dominant expansion: it is not tonic in function.
Ex. 7.9 - the cadential six-four occurs at b. 144; the six-four chord at b. 148 is a neighbouring six-four expanding the closing tonic plagally.
Ex. 7.10 - the cadential six-four occurs at b. 10; the six-four chord in b. 93 is not cadential but arpeggic (Ex. 7.1 & 7.2), i.e. the first three beats of b. 9 is tonic in function, leading to the PD ii6.