The boiled down message of the post was that almost everything is uncertain, and that uncertainty manifests in different ways at different scales, but nonetheless arises at all scales. It’s true, as you say, that if you’re in a perfect simulation and will always be then this makes no practical different in terms of your experiences of reality (assuming you can’t figure out that you are in one). But even if it has no practical ramifications, you of course still could be in a simulation.

One thing that is problematic about direct experience is that they constantly change. By the time you think “I had the experience of seeing a blue cube” that is only a memory, and memory is fallible. So reflections on direct experiences admit some possibility of error. But direct experiences themselves, the moment you have them, might be an example of something not to be doubted, as you say.

You missed the largest class of 100% certainty beliefs. Every direct conscious experience is 100% certain.
“If your mental experiences are varied…” How do you know they’re varied? Perhaps they’re all the same and you’re just mistaken. The answer is that’s impossible. The reality of what you’re experiencing is determined by what you’re experiencing, by sheer law of identity. If I perceive a red apple, then a blue cube, I can only justify uncertainty about what I infer from those perceptions. Notably, the perception of the cube as different from the apple is itself a direct perception.