@dalehileman,
A lot is implied when we say "as agreed." You could "spell it out" in greater or lesser detail if you wanted. Basically it acts as a shortcut, or contraction.
1. As (previously) agreed.
2. As (they had previously) agreed.
As suggested, here "as" really serves to fill in for a longer phrase such as "in accordance with."
3. In accordance with the terms of their prior agreement,.....