RTF is a global modification that replaces all towns in the game.

But does the game support two different attack animations? I mean, specifically the "attack" string. So, when you click "attack," does the game randomly choose one or the other animation?

Yes, stir can also be double.
They are marked as attack00 and attack01.

Double attack
For example, a beast bites and kicks.
A knight swings vertically and horizontally.
A minotaur swings one axe vertically to the ground and two horizontally.
A cave lord does the same; the list could go on, and almost all dragons have two animations.

In the add-ons, units almost always have one attack animation; I think only the goblin shaman hits with its head and foot, but upgrades often have different animations.

Stir00 is almost always single, but the knight hero has two animations - one when the knight looks around, and another when the horse shakes its neck... or twitches, I don't know.