They're pretty clearly taking liberties with what constitutes a horn when assigning the move to Pokémon but the Japanese name really is "Smart Horn" in katakana.Afaik Smart Strike is not strictly linked to "horns" but rather to perforating ability (which well... you'd perform with a horn). It's why certain Pokemon that can't learn megahorn can learn it, like Cloyster, Togetic or Tapu Fini.
Kartana being based on sharp paper definitely has the pointy appendages to perforate something.
The localization for the move's name also reference sharpness, and "Smart Strike" despite being a weird name could also be implying the idea of a precise stab.
Basically Megahorn would assume the user rams itself with all its force using the horn as focal point for the attack, while Smart Strike is rather a precise stab with a pointy edge (generally but not necessarly a horn).
As stated above, feels like Aggron lacking Megahorn may just be a oversight, as other pokemon with relatively small horns like Seaking are able to learn the move.
It's a bit of an odd move in general; not sure why piercing attack with a horn that always hits is Steel-type. Maybe it's using the horn as a radar to home in on opponents?