Solomonoff’s only assumption is that the environment follows some unknown but computable probability distribution. Everything else is a mathematically proven theorem which come from this only assumption, and the traditional axioms and definitions of Math.
So if for you “the map” means “follows some unknown computable probability distribution” then you are right in the sense that if you disagree with this assumption, the theorem is not valid anymore.
But a lot of scientists do make the assumption that there exists at least one useful computable theory and that their job is to find it.
Kilobug wrote “There is no updating of probabilities, because hypothesis are always right or wrong” Do not forget that any wrong hypothesis can become right by adding some error correcting instructions in it. It will only make the program longer and thus lower its probability. But is seems intuitive that the more a theory needs error corrections, the less it’s probable.
Kilobug wrote “there is no room for an hypothesis like “the coin will fall heads 75% of times, tails 25% of time” There is room for both an hypothesis that predict the coin will always fall heads and this hypothesis has a probability of 75%. And an hypothesis that predict the coin will always fall tails and this hypothesis has a probability of 25%.