There's nothing the least bit implausible about the idea of two different creators independently coming up with the same idea. It happens all the time. Fiction is like language -- it has its own syntax and vocabulary, and there are only so many ways to put the parts together into a coherent whole. So different creators often end up putting ideas together in fundamentally similar ways. This is why the vast majority of plagiarism lawsuits get thrown out. Actual plagiarism by professional writers is extremely rare, because most professionals know how stupid it would be to attempt theft in a field as fundamentally public as film, TV, or prose. Most of the time someone's accused of plagiarism, it's actually just a coincidence.
And the fact that Ellison's story was written decades ago doesn't prove anything. None of us has read everything ever written. There are plenty of famous writers whose work I've never read. So it's not unlikely at all that Andrew Niccol was unfamiliar with Ellison's work.