If I am creating an application which is critically dependent on a good quality TRNG, can anyone see any reason why it might be a mistake to depend on this script for the purpose?

Well, aside from being too simple, and depending on simple counting loop for sleeping .... to induce a difference

If you go to TRNG they talk about dealing with bias ...

In short, if you have to ask if yours is a good quality TRNG, it probably isn't :)

But thats just a feeling I have (smell test) ...

I'd seek out some kind of cryptologist mailing list or something :)

Good luck