No. An argument that addition of integers is commutative will not contain such a qualifier. Nor will an argument that the laws of general relativity, say, correctly describe gravity in the regimes we have tested. If we had an argument of that sort--an argument based on deductive logic or detailed empirical knowledge of the problem domain--for a conscious computer being impossible, it would not need the qualifier either. But we don't.
That sounds a lot like the qualifier so far as we know in your own example. In fact, the only example you've given where it arguably doesn't apply is mathematics.
David Hume famously argued that causality is based on experience, and experience similarly based on the assumption that the future models the past, which in turn can only be based on experience – leading to circular logic. In conclusion, he asserted that causality is not based on actual reasoning: only correlation can actually be perceived.
No, because the claim that I made was limited in the same way as the qualifier: I didn't claim that GR was valid period, I only claimed it was valid in the regimes we have tested. But in those regimes, it is valid, period, or at least that was my claim--even though we haven't run all imaginable experiments in that regime.
The problem with the original claim I responded to is that it wasn't limited that way: the claim was made that something is impossible, period, but the supporting argument had the "so far as we know" qualifier in it. So the claim was going beyond the regime in which the supporting argument applied.