explanation?

Actually, the meaning of the sentence is that someone uses a double standard, NOT a clear line of thought.

One day says yes one day say no. Or in public says one thing and private another...and so forth

Which means that his behavior is not impeccable.

E is the best

Ask if something is still unclear.

Regards

The correct answer is detached and aloof

but it's not a not sentence equivalence question

It is a one blank text completion question and not sentence equivalence.

It is a SE but the user used only one answer

It is a two answer Sentence-Equivalence question from Manhattan GRE, and has two answers. Original post's correct answers need revision.

Official explanation MGRE:

To expatiate on a topic is to “talk or write at great length on a topic.” In this case, those who “resist the temptation to expatiate on self-indulgent topics” are actually quashing their desire to talk about themselves. “Overcorrecting” in this manner would be to go to the opposite extreme, likely talking little or not at all. Thus, these people might be perceived as overly quiet or reserved in a negative way. We’ll use “overly reserved” as our filler.

(A) Loquacious means “talkative.” This is the opposite of what we want.

(B) CORRECT. Detached, when used to describe a person, means “disinterested, unconcerned” in a negative way. This is a good match.

(C) Communicative means “inclined to talk or communicate.” This is the opposite of what we want.

(D) CORRECT. Aloof means “disinterested,” and generally in a negative, almost arrogant manner. This is another good fit.

(E) Irreverent means “not showing respect.” This does not match our filler.

(F) Reprehensible means “detestable” or “blameworthy.” It’s a negative word either way, but it doesn’t mean “overly reserved.”

Notice that (A) and (C) make a synonym pair, but with the opposite meaning of what we’re looking for.