they used this script:
-- Revert kleis attack canceling fix
AddComponentPostInit("projectile", function(self, inst)
self._ondelaycancel = function() --[[disabled]] end
end)
The one Chinese dude made the mod that allows you to attack much faster than intended. If Klei wanted to do, they would have made this fix long ago.
anim cancelling is not an exploit or bug. It exists in many games, and is often even an intended gameplay mechanic to reward players who do things manually.