All I know is that the people who serviced our sprinklers at the old
house had a BIG compressor It just fit in the bed of their pickup.
They kept air on the pipes until no more water sprayed out, which I
would assume would take more air than a pancake could generate.

So a pancake or shopvac might be able to push water out of the system,
but that doesn't mean it's getting all the water out, or even enough
to prevent ice damage if the remaining water collects in a low spot.