There is probably a cloud containing nothing but radon (the heaviest elemental gas) somewhere in the universe as well, right? If that exists would it disprove the big bang, or would it simply have been there by chance for billions of years, just like this one could have been?
Hydrogen and helium isotopes (and a little bit of lithium and beryllium) are made in the Big Bang. Everything heavier is made in stars. So these pure clouds can exist only as long as there are no stars nearby to pollute them with heavier elements. Stars are common in the modern universe, which is why it has been so hard to find such clouds.
Radon in particular is made in supernova explosions (and by the decay of radioactives which were made in supernova explosions) and there is no natural mechanism to separate it back out from mixtures of supernova debris. So in a sense, yes, if a massive, primordial, pure radon cloud was out there, it would disprove the Big Bang theory's prediction of nucleosynthesis, which can only make light elements.
But ASL has different grammar and syntax than spoken English. It's closer to signed French.
Numeric stability is probably not all that important when you're guessing.