Alluding to Nika's existence is not the same as foreshadowing Gear 5. For instance, vaguely referencing Nika is not the same as implying that Luffy "is" Nika. I do genuinely believe that Enel was trying to copy Nika, though. Again, I'm critiquing the fanbase, not the series itself.
While Oda may not have had Gear 5 in mind when he created Skypiea, he knew that Nika would exist in some form. The themes of gods and freedom appear throughout One Piece, as seen with the Lunarians, who were "cast down from the heavens," and the Celestial Dragons, the "divine" oppressors. Therefore, this specific instance from 20 years ago is more correlated to the overarching themes of One Piece rather than to a particular transformation.
P.S. I’m not saying Gear 5 didn’t have foreshadowing; I’m saying that the explicit foreshadowing it did have was not from 20 years ago. All writers know that stories change and evolve, and to maintain the same notion and idea for 20 years regarding something so minute and specific, like a transformation, is not something writers are capable of doing. Oda is only human, after all.