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Basically, the NX bit prevents CPU behavior (speculative fetches) that had a hand in Spectre-type vulnerabilities. That's surprising because that's not its purpose. This is for ARM CPUs.

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No, NX precedes Spectre by a long shot. It was originally intended so an attacker couldn't use a buffer overflow to change the PC and execute directly out of the attacker-controlled buffer.

That is exactly what the poster to whom you replied said.

So Arm did not add another means to disable this kind of speculative execution, after Spectre was discovered, but they just reused the existing NX flag, expanding its functionality.

That poster wrote about speculative vulnerabilities. But buffer overflows have nothing to do with those.

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