Obviously, Chloe's Eevee is Gigantimax. That's why she doesn't evolve, despite it touching stones to try. And Pikachu wouldn't have evolved even if Ash tried, because GMax Pokemon don't evolve.
I agree with your opinion as initiallly I also thought that and posted the same here. However, I think there's also a Possibility that they might introduce a new Eeveelution considering the fact that it was mentioned several times that the Eevee was unique and will probably take a different path....