Re: "what about my argument that a human can [adapt its razor a little] and thereby do better than a Solomonoff predictor because the Solomonoff predictor can't incorporate this?"

There are at least two things "Solomonoff predictor" could refer to:

• An intelligent agent with Solomonoff-based priors;

• An agent who is wired to use a Solomonoff-based razor on their sense inputs;

A human is more like the first agent. The second agent is not really properly intelligent - and adapts poorly to new environments.