I'm hoping serebii is wrong unless its because links doesnt involve snorunt by the time Aya evolve. But why is it only snorunt and froslass that are required and not glalie like most other pokemon with their evolutions?
Because her special 100% link Pokemon is Froslass, not Glalie.
And this is the only case of that with any main warlord/junior warlord in the story. Aya is the only unique warlord with a 100% Pokemon that evolves like that.
There are also some generic warriors who have split-evo perfect links, where they only have the perfect link with one top form. In those cases, they will have all of the pre-evolutions too but not the other final evolution. Example:
Yoshitsugu has a perfect link with Gardevoir. This means that he also has Ralts and Kirlia as perfect links, but not Gallade.
Kagekatsu has a perfect link with Gallade. This means that he also has Ralts and Kirlia as perfect links, but not Gardevoir.
Aya's fully-evolved perfect link is Froslass. Froslass just happens to be a split evolution. So she has a perfect link with its pre-evolution (Snorunt) but not the other final evolution (Glalie).
Just equip a Dawn Stone on her while she's using Snorunt and fight a battle to get Froslass.