Hello, owner of the site here. The exact reason the page says that is because the Pokemon is new, I haven't added any evolution chart yet (plus the evolution method is not known). So it just says that message by default. We know it evolves from Eevee but we don't know of any other evolutions and such yet.
Note at the top it says, This page is about a new Pokémon; details may still be sketchy.
Anyway I changed it a little while ago to say, The full evolution chain of Sylveon is not yet known. So that should be a bit clearer.