No, this is not just a linguistic disagreement. This is the crux of the matter. By saying that "the earth is round" and "the proposition 'the earth is round' is true" mean the same thing you are conflating two different ontological categories. When you do that, your reasoning is no longer sound.
Consider this:
P1: The U.S.S. Enterprise can travel faster than light.
Is P1 true? If yes, then how can that be when we know from relativity theory that nothing can travel faster than light? And if no, then what about these:
P2: The U.S.S. Enterprise is powered by a matter-anti-matter reaction controlled by dilithium crystals.
P3: The U.S.S. Enterprise is powered by squirrels running on treadmills.
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P.S. It occurred to me that there is a TL;DR answer to the question of what Turing did that was so important: he invented general-purpose software.
No. What I deny is that "the earth is round" and "the proposition 'the earth is round' is true" are identical propositions. The first is a proposition that makes a claim about the roundness of the earth, and the second is a proposition that makes a claim about the truth of a proposition. Those aren't the same thing.
The earth is round whether or not anyone is around to contemplate it (the earth). But "the earth is round" is not true unless someone is around to contemplate it (the proposition).
> What did Turing do that proved that propositions only exist insofar as they are conceived?
That's a long story, and I have to run to a meeting right now. If you really want to know, remind me later. I may need to write a blog post about it.
But forget about propositions for the moment. Do you think that unwritten poems exist?