Existing in an un-simulated reality

This reminds me of the "dust" theory in Egan's Permutation City.

It is indeed similar.

It seems to take a highly reductionist pathway: reality/experience can be simulated -> simulation is computation -> computation is mathematics -> mathematical objects exist regardless of whether anybody has discovered them.

This implies that all conceivable universes (including the ones where a lot of really bad things happen on an eternal loop) are possessed of the exact same reality as ours.

One one hand it's absurd (it means that everything that can be imagined and many more things exist).

On another hand the opposite (requiring a mapping from that computation to real-world objects) is absurd too, because for any sequence of numbers you can always find a mapping to physical objects (notice that you can make the mapping arbitrarily complex). So why require the extra steps?

My opinion is that it follows that asking about existence without specifying the domain in which sth exists is meaningless.

You can say that the number 42 exists in the domain of integers. You cannot say whether the number 42 exists in general. It wouldn't mean anything.

Similarly you can say that Harrison Ford exists in our universe but Han Solo doesn't. But you cannot say whether one or the other exist in general.

Am I missing something here? I can suppose a universe where the premise is false (This is a conceivable universe, I’d argue). Doesn’t that mean that this premise really is false?

I believe that you are more real than Donald Duck if and only if the future is not yet determined.

In philosophy, theories of time are categorized in A- and B- theories [1].

In B-theoretic time, the difference between past, present and future is only subjective. Objectively, all points in time exist and are equally real. I view this to mean that there isn't any particular difference in the degree of reality between our reality and any mathematical model or imagined reality. Only with A-theoretic time are you objectively more real than Donald Duck.

[1] https://plato.stanford.edu/entries/time/#TheoBTheo