During the Enlightenment, which was the heyday of deism, philosophical proofs of God's existence were still taken seriously by most deists. So they weren't agnostic at all--they claimed to know, based upon reason, that God existed. A perfectly consistent Enlightenment-era viewpoint.

However, since they only go as far as "God exists" and never get to the more exotic beliefs, namely the beliefs which always were and always will be taken on faith (God had a human son, who was also God, died, and was resurrected; God chose a Middle Eastern ethnic group for a special purpose; God revealed prophecies to certain people; God is from outer space) they weren't religious.