If the map was generated and immediately launched in-game (as is always done), then the artifacts in Dragon Utopias and other things determined upon launching the map can be discovered via the seed. This requires certain reverse-engineering skills (more substantial than simply using artmoney), but the task cannot be called very difficult.

Most likely, the guessing happened exactly via the seed; I see no other reasonable options in terms of difficulty.

The seed could be removed from the map description; then restoring it from an open section of the map would be nearly impossible, and neither the client nor third parties would be able to find it. However, the host generating the map can still find the seed (if we assume they know how to restore a map via the seed, this is not a problem for them).

One could first generate the map, then exit the game and launch it from the saved generated maps. In that case, artifacts in Dragon Utopias would not depend on the seed. But still, restoring the layout is not very difficult for the host. Essentially, protecting against "pre-game" cheating by the host is practically impossible if we assume they possess certain reverse-engineering skills.

Added after 7 minutes
UPD: Although, perhaps the option of launching a saved generated map is impossible: the map is password-protected, and I don't know for sure if it can be launched online (it cannot in single player or the editor), I've never tried.