I wouldn't say it's quite the same as the Blue's Raticate theory since that theory is more less making something from nothing. This may be just one line that Zinnia says, though hers is actually direct in regards to what people think it means and has an overall more significant implication on canon if it's true (without actually disrupting the canon either).While I can't pinpoint the exact reason why it's overblown, something's telling me it may be a similar reason to the 'Blue's Raticate died' thing - overanalysis of a couple of lines/a minor change to the point where it's treated like a major plot point when in reality it doesn't mean that much to the canon as a whole.
I think this is more of a case of "it could mean exactly what people think it means" or it doesn't, and I really can't find anything that makes either side objectively more right or wrong. There's probably no way to know for sure unless they reference it again in the future but I don't think it's conclusively true to say her line means nothing and everyone is overreacting. It could be the case that her words meant nothing but I think it's equally likely that this "multi-universe" is a legit thing they're trying to hint at. They dropped that line so casually that I don't see enough grounds to rule anything out.
For all we know they put that line in specifically for us to do exactly what we're doing now, discuss the significance of what she implied about multiple universes. And purposely leaving it ambiguous without ever touching upon it again in order to further it as a discussion point would be very clever of them.