I suspect that the internal VP structure that Carnie presents in (28) corresponds to an early 1990's analysis of the 'event structure' of active VPs according to which such verbs contain an abstract ('do-like') 'active' component which is absent in the case of verbs describing non-agentive events (e.g., 'processes' like rain, bare 'change of state' verbs like die, or non-active 'accomplishment' verbs like reach in reach the summit, etc.).
The basic idea is that the Agent of active verbs like eat is projected as a specifier of that [ACT] V head (i.e., under VP1) - instead of as a specifier of eat itself (under VP2) - or, more intuitively, that eat is the surface manifestation of a complex predicate meaning 'do eat' or 'perform an eating act'.
Although that analysis immediately raises difficult theoretical questions like what, then, if anything, occupies the specifier of eat under VP2, or, if he is indeed first projected as the Spec of eat under VP2, what kind of thematic role it receives from eat and how can it subsequently raise into the specifier of the 'do-like' V under VP1 and receive a second? theta role (i.e., that of Agent) there, and why is that compatible with the Theta Criterion, etc., the fact is that there is some empirical evidence in support of that analysis.
Pairs like * What you didn't was educate your son properly vs. What you did´t do was educate your son properly show that there is a do verb 'hidden' under active verbs that is different from the 'dummy' do auxiliary and shows up only in such contexts. The fact that non-agentive verbs like die, see, etc. cannot be used in that construction (cf. * What he didn´t do was die/see the accident) suggests that they do not 'contain' such a hidden 'active' do verb and therefore lends support to abstract analyses like Carnie's (initially due to Hale & Keyser, if I remember correctly, but very popular in the late 1990's; see e.g. Malka Rappaport Hovav & Beth Levin's influential 1998 paper 'Building Verb Meanings').