It sounds like 170,000 is every possible combination of actions that might ever be valid. They stated that usually around 1000 are valid at any point in time.

Based on the examples under the "Model structure" section, I'm guessing they are counting all combinations of spell and target location, including locations on the ground for ground-targetable spells? That could add up quick... e.g. 10 spells * 20 target units * 9x9 grid of locations around each = around 16,000 possibilities.