coupled though it be is a concession clause with subjunctive form of BE.
though it be coupled with fear
although it be coupled with fear
coupled though it be with fear
Introducing the concession clause with the subject modifier coupled gives that notion a slightly more prominent place in the clause.
The sweater, damaged though it was by moths, would still keep a person
warm. without subjunctive
The narrow road winding up the side of the mountain, dangerous though it be, is your only option. with subjunctive
The subjunctive here is somewhat old-fashioned. It has a literary feel to it, rather than a conversational one.
P.S. A comment of mine was deleted by someone to whom rules are more important than politeness, so I will add it to the answer.
The phrase in the original which results in the he to whom structure is the idiom "X is a stranger to him", meaning "He does not know X" or "He is unfamiliar with X". To be a stranger to s.o.
The main verb phrase is "is as good as dead" and so the subject is the person who has not experienced the mysterious, he, that person, is as good as dead.
He to whom this emotion (i.e. the sense of the mysterious) is a
stranger...is as good as dead.