What I'm saying is there might be a difference between the computation of a thing and the thing itself. It's basically an argument against the inevitability of Tegmark IV.

I think you mean, it's the skeleton of an argument you could advance if there turned out to actually be some meaning to the phrase "difference between the computation of a thing and the thing itself".

Or at least I've not been convinced that it's a logical impossibility for it to be otherwise;

Herein lies the error: it's not up to anybody else to convince you it's logically impossible, it's up to you to show that you're even describing something coherent in the first place.

Really, this is another LW-solved philosophical problem; you just have to grok the quantum physics sequence, in addition to the meanings-of-words one: when you understand that physics itself is a machine, it dissolves the question of what "simulation" or "computation" mean in this context. That is, you'll realize that the only reason you can even ask the question is because you're confusing the labels in your mind with real things.