Open trigger vs. Closed in Anderson lowers doesn't have anything to do with the trigger itself. "Closed" means it has an integrated trigger guard forged in with the lower, whereas a standard AR-15 lower has an "open" trigger guard, requiring a pin-in-place trigger guard.
So you could put a Timney trigger, or a better trigger, into either an open or closed Anderson lower.
Note in the pics below, only difference is the trigger guard, one is closed, one is open.
Here's a "closed" Anderson lower:
Here's an "open" Anderson lower:
