If this would have happened in modern times, with cameras at the scene, there might be validity to this question.
However, a mention of this situation in a 20-century old book has no hope of ever being valid as evidence, let alone proof (*any* 20-century old book).
Then still, even *if* this mass revelation really happened, and it was indeed a god speaking, this still only indicates that judaism is true, not that it is the only true religion; who claims that there is only one "true god"? many societies had several gods, even christianity hints at this. "Thou shalt not worship any god but me", why say that if there is only one god? Jaweh basically stated "yeah, there are more gods, but i'm jealous, so worship only me".
i can imagine judaism and islam have similar passages in their books