teenager | mathematics enthusiast | Grey Triber | sometimes philosopher | personal website: https://duck-master.github.io
Au contraire, I think that "mutual information between the object and the environment" is basically the right definition of "knowledge", at least for knowledge about the world (as it correctly predicts that all four attempted "counterexamples" are in fact forms of knowledge), but that the knowledge of an object also depends on the level of abstraction of the object which you're considering.
For example, for your rock example: A rock, as a quantum object, is continually acquiring mutual information with the affairs of humans by the imprinting of subatomic information onto the surface of rock by photons bouncing off the Earth. This means that, if I was to examine the rock-as-a-quantum-object for a really long time, I would know the affairs of humans (due to the subatomic imprinting of this information on the surface of the rock), and not only that, but also the complete workings of quantum gravity, the exact formation of the rock, the exact proportions of each chemical that went into producing the rock, the crystal structure of the rock, and the exact sequence of (micro-)chips/scratches that went into making this rock into its current shape. I feel perfectly fine counting all this as the knowledge of the rock-as-a-quantum-object, because this information about the world is stored in the rock.
(Whereas, if I were only allowed to examine the rock-as-a-macroscopic-object, I would still know roughly what chemicals it was made of and how they came to be, and the largest fractures of the rock, but I wouldn't know about the affairs of humans; hence, such is the knowledge held by the rock-as-a-macroscopic-object. This makes sense because the rock-as-a-macroscopic-object is an abstraction of the rock-as-a-quantum-object, and abstractions always throw away information except that which is "useful at a distance".)
For more abstract kinds of knowledge, my intuition defaults to question-answering/epistemic-probability/bet-type definitions, at least for sufficiently agent-y things. For example, I know that 1+1=2. If you were to ask me, "What is 1+1?", I would respond "2". If you were to ask me to bet on what 1+1 was, in such a way that the bet would be instantly decided by Omega, the omniscient alien, I would bet with very high probability (maybe 40:1odds in favor, if I had to come up with concrete numbers?) that it would be 2 (not 1, because of Cromwell's law, and also because maybe my brain's mental arithmetic functions are having a bad day). However, I do not know whether the Riemann Hypothesis is true, false, or independent of ZFC. If you asked me, "Is the Riemann Hypothesis true, false, or independent of ZFC?", I would answer, "I don't know" instead of choosing one of the three possibilities, because I don't know. If you asked me to bet on whether the Riemann Hypothesis was true, false, or independent of ZFC, with the bet to be instantly decided by Omega, I might bet 70% true, 20% false, and 10% independent (totally made-up semi-plausible figures that no bearing on the heart of the argument; I haven't really tested my probabilistic calibration), but I wouldn't put >95% implied probability on anything because I'm not that confident in any one possibility. Thusly, for abstract kinds of knowledge, I think I would say that an agent (or a sufficiently agent-y thing) knows an abstract fact X if it tells you about this fact when prompted with a suitably phrased question, and/or if it places/would place a bet in favor of fact X with very high implied probability if prompted to bet about it.
(One problem with this definition is that, intuitively, when I woke up today, I had no idea what 384384*20201 was; the integers here are also completely arbitrary. However, after I typed it into a calculator and got 7764941184, I now know that 384384*20201 = 7764941184. I think this is also known as the problem of logical omniscience; Scott Aaronson once wrote a pretty nice essay about this topic and others from the perspective of computational complexity.)
I have basically no intuition whatsoever on what it means for a rock* to know that the Riemann Hypothesis is true, false, or independent of ZFC. My extremely stupid and unprincipled guess is that, unless a rock is physically inscribed with a proof of the true answer, it doesn't know, and that otherwise it does.
*I'm using a rock here as a generic example of a clearly-non-agentic thing. Obviously, if a rock was an agent, it'd be a very special rock, at least in the part of the multiverse that I inhabit. Feel free to replace "rock" with other words for non-agents.