What? That makes no sense either. "One life, one death, there is but one God". It acts exactly like an Abrahamic religion, even down to the part of there being only one god and all other gods being false, a singular being who made everything. That is basically the complete opposite of Deism. I suspect you're wrong about this and have no evidence at all, but we'll see.
I think I can answer that - while Deism doesn't really describe the concept well, it has a very important aspect that Andrastianism originally preached: there are no (more) miracles. Prayer doesn't work. God exists, but he won't lift a finger.
This aspect of religion got watered down A LOT in further installments, but originally was quite defining. The official stance of religious authorities was that Maker is mad at the world and doesn't intervene in any way now. No miracles will occur and no prayers will be heard until people get their sh*t together and start singing Chant of Light in every corner of the world.
I'm pretty sure that's what In Exile meant when he mentioned Deism - the idea of absent God that does exist in Deism, even if other aspects are obviously at odds with Andrastianism - starting from the fact that Deism generally opposes the idea of organised religion, especially if the latter claims to be started through supernatural interventions (like, say, being started by God-chosen miracle-making prophet).
In fact, if we're talking about Thedas, Imperial Chantry is a bit closer to Deism, as it, generally speaking, rejects most miracles, doesn't really acknowledge Andraste as bride of the Maker and possibly doesn't even include Drakon's revelation in its Chant of Light (and that's where the idea of spreading Chant of Light to reclaim Maker's grace comes from). This takes a lot of supernatural from their religion, bringing it relatively close to philosophical principles of something called Christian Deism.