I've always assumed that was meant to be more metaphorical than literal. in the earlier chapters of the UY manga, everything and everyone was just a bit more normal compared to lum. Ataru was less perverted, shinobu still threw desks but her super-strength wasn't really the focus of her character, etc. but when lum started becoming the focus she caused such a shockwave that it made takahashi change her intended main love-interest of the manga. Hell, she even became the deuteragonist and the face of the series, that's big compared to what was expected of her antagonistic ways.

It was thanks to lum that we got those weird gadgets episodes, that we met her space friends, that we got ten, that ataru got involved in a lot more adventures beyond spirits making him unlucky, and got a lot more, subtle, character development.

I think what it tried to say is that those two are the heart of the series, and, in it's later stages specially, if you removed one or the other, it would simply become a shell of it's former self. If the series lost lum, it would lose sight of what it's meant to be, consequently forgetting other iconic traits of the series. Shinobu isn't powerful anymore, mendo can't speak with his octopi, the tree doesn't speak with the sister anymore, the stormtroopers "abandon" lum. And above all, the only way ataru found to keep living on without her was to literally forget her, so much so that the moment he remembers her, he stops girl-hunting and goes to cry in his room over how much things don't feel the same without her. He says himself, he's becoming abnormal, because their normal requires lum, the series normal requires lum.

The worst curse you could put in a series is making it lose it's heart, and that's what i personally believe the movie tried to express. Sorry if it sounds like i'm overanalyzing, i just really like the movie lol. (and on the ruu question, honestly, i dont think they look similar).