Although there has been explained, in fact, I cannot yet get why the bold part has been used. And, I am wondering if a and b mean the same thing,if so when? when not?
In addition, what are them? are they considered as conjunction?
a. let alone: used for saying that something is even less likely to happen than another unlikely thing: I hardly have time to think these days, let alone relax.
b. still/much/even less: used after a negative statement in order to emphasize that it applies even more to what you say next: I am no one's spokesman, much less his.
You could make the case that let alone and much less are synonymous, since you could substitute one for the other:
I am no one's spokesman, let alone his.
I hardly have time to think these days, much less relax.