I think you're right that he used the term "higher being", but I am as certain as I can be that he was not advocating the necessity of such, but only allowing it into hypothetical consideration after being pressed by Stein.
I think he did say that anyone that was capable of doing this must be operating on a level far beyond humans, and I think this is what you are remembering. The "must" is applied as a stipulation in the hypothesis, and by no means refers to a belief that the hypothesis "must" be true.
The point was that he was incapable of explaining the origins of life and had to resort to “intelligent design” despite himself. It was a beautiful moment.