The amount of useful cooling provided by thermosyphon can’t exceed heat soak because the energy for the former comes from the latter.
If you look at the graph on the heatsoak thread, Solving heatsoak - a worked example, then you’ll see that even running the fan does little to abate post shutdown temperature in the cylinder head (the light blue trace still goes up, but perhaps at a lesser rate, whilst the dark blue rad temp doesn’t come down particularly fast until the fan is deployed), but it does a good job of creating a big reservoir of cool water in the radiator, which can then be deployed and which does make a difference.
kind regards
Marek