t sounds, to me, like the writers are, once again, guilty of lack of world building. They need to save money so they can’t really build an entire set, with heavy CGI, for the Underworld so they use their already built SB set and a red filter.
Rumple’s line was an example of shoddy writing, yes. On the other hand, it doesn’t bother me too much that the UW is Storybrooke with a red filter. I think it’s actually preferable to yet another cheaply cobbled together CGI realm.
An easy solution would have been to have Rumple say something to the effect of the UW appears to different people as different things, and it’s appearing to them as SB because [insert reason blah blah].
Anyway, I agree with your larger point. A lot of the world building of UW is, so far, pretty muddled.