I also feel like many of its followers arent so much into it as a fetish per se - it's more just a way of saying 'I like harems'.
How very politician of you to say "many"... You running for office?

Harem in the sense your SO allows or wants you to have sex with others, is pretty much required in order to have a cuckquean... So, yeah, of course cuckquean-harem can be linked, and I'm sure many are weighted toward 'harem' more than the cuckquean side.
The cuckquean appeal (for me) overrides the "harem" fantasy.
Your wife/gf thinks you're so...impressive...that she wants to show you off, or please you. Obviously male ego thing...
That she has confidence she'll always be your #1 no matter what...
And she actually gets off on it...
Actual cuckquean is a very rare story line in VNs. So the majority options are "harem" where your SO is just ok with MC having sex with others, she doesn't necessarily get off on it or encourage it.
There are so few actual cuckquean stories, so there's not a large enough sample size to say most (or even many) cuckquean followers are leaning more toward saying 'I like harem' over the appeal of the wife getting off on "sharing" the husband...
Now, the other part of cuckquean, where the MC/husband/BF wants to 'humiliate' the FMC/wife/GF or cheat on her, yeah, I'd agree the majority of those are saying 'I like harems' and she needs to be okay with MC having sex with others. I don't like cuckquean 'humiliation' like that. If she ain't ok with it, I don't want it. But hey, if she's into the 'humiliation', sure, I'll play along. But I'd also want to go out of my way to make sure she knows it's just 'play', and not my actual feelings...
any way, TLDR...I do agree many who like cuckquean do lean toward it being another way to say I like harem, but I'm not sure it's most or even a plurality. I'd almost go so far to say those 'I like harem' folks don't really like the cuckquean fetish at all, because it's more to do with the wife/gf than the husband/bf, where harem is more husband/bf focused...