I think it's only eternalism if someone tries to view it in that way.
Just consider the following: there are some people who say that the Buddha taught nihilism. Does that mean that the Buddha himself was a nihilist? Of course not... it's just these people who have a nihilist view of the teachings, even when they claim themselves to not be nihilists
. That kind view is very subtle... but still quite significant. It's a part of what causes the dukkha. I think it's exactly this sort of thing that the Buddha taught about the wrong views, and also about what the right view would entail.
It should be really obvious to anyone who practices anyway, that a jhana always end sometimes... or to put it more accurately, they haven't yet encountered a jhana that is permanent... or else they wouldn't be going back to the sitting, secluding themselves, etc. There's no way to not realize this, unless you're really delusional about it. It doesn't really matter what kind of explanation you give it... it's still all a sign of impermanence. It's due to dependent origination... the moments have nothing to do with it.
"Moments" don't really bother me, though... I've argued very strongly against "flux" somewhere on this forum before. (Anicca might describe flux, but that doesn't mean the anicca is a flux in itself.) That doesn't mean that my opinion is changing between these two... it's just that when I think I see someone is attaching his own views to either one of those... I'll try to say something about it. Neither of those two has anything to do with the Dhamma, and therefore aren't necessary for practice. Neither of those will give you any insight about the view-clinging, anyway.