I'm familiar with JC (er.. Jos. Campbell too ;-)
Not sure if you see the implications, since you appear surprised at both Campbell's view and that of the 'most people'. By most people - I will presume too, that you mean literalists / Fundamentalists (?) since from that POV: it is unfathomable; opposes all their castles in the air and the contents of those castles.
Simply read those words with comprehension. What else are they saying than ~~ a stark statement that this 'Kingdom' is not of space or time: with massive inferences for those who become bogged down in history, translations, 'bodies' and literal interpretations of all those metaphors. No 'place' is needed, nor past/present/future (human concepts all).
I think you'll find that the esoteric wings of Christian sects (Jesuits for one, at the top of the hierarchy) share with the same wing of most other - even Corporate - religions, such a view of "It All".
The Bible, like many equivalents in the world - tries to cover all bets; simplistic homilies for beginners and hints for the advanced - who ~grok the 'nontemporality' idea and wish to know more. Those who see the parables as Sufi sayings (!) and understand what a metaphor is: will read a quite different book from what the literalist reads and embellishes from imagination.
Which is perhaps why all the wars start - either internecine ones, sect <> sect, or on through swords and stuff. Only the simple-minded find excuses for wars in these common human documents. Those who actually want to know - are aware of the limitations of language and don't give a Damn about the angels dancing on pins / all the other contradictory material [except for the fun of argument, that is: See Monty-P - The Holy Grail, etc]. :-\ufffd
Anyway - not merely insignificant-I, but many others see this exact 3-sentence-sentence - as encapsulating that common Truth which defies simplistic deconstruction into bite-sized slogans. (Never mind the Xian hubris of imagining theirs the Only Truth - or even that they found it first! .. and all the usual tribal BS)
It's a gem, wherever found. Pity there's so much scabrous nonsense fore and aft -- with merely the occasional jewel left unedited or expanded into meaninglessness. Screw the apocalyptic folk! Apoplexy is what they yearn for - let them enjoy their delicious waiting .. Of Course "they don't get it" ! For all mentioned reasons.
IMhO you cannot 'explain' these 3 sentences to a retard; it just won't happen. You have to find out a lot - and then dismiss that all! before you can get close.. how about calling it a most difficult Simplicity?
Hang in there. The Cosmic humor seems most evident: in the amount of adulterated BS you have to wade through then ignore!, before ever even getting close to, what must be ultimate Simplicity. (For penultimate humor, add in especially the conundrum: I Know.. and I'll Tell You !!) as in..
Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle.. Cackle..
As God 'says'
I Am What I Am