Subject-determiner

This is from "A Student's Introduction to English Grammar" by R. Huddleston and G. Pullum (p.255):

"Non-dislocated clause

[42] ii a. I think the man next door's car was stolen.

Dislocated clause

[42] ii b. The man next door's, I think his car was stolen.

Left dislocation may put a complex NP early in the sentence, replacing it with a pronoun in the nucleus [a main part of the clause, consisting of subject and predicate], so the nucleus is structurally simpler. (Note that in [42iib] the subject-determiner in the dislocated version is simply his, whereas in [42iia] it is more complex genitive the man next door's).

My questions are:

Why is the genitive the man next door's the subject (called subject-determiner here) in the subordinate clause of the sentence I think the man next door's car was stolen instead of the ordinary subject, an NP, i.e. the man next door's car?

Why is the determiner his the subject (called subject-determiner here) in the subordinate clause of the sentence The man next door's, I think his car was stolen instead of the ordinary subject, an NP, i.e. his car?