I probably didn’t explain myself well enough, or maybe there’s a deeper disconnect between our views. Imperfection and limitation are basically synonyms, something is imperfect if it fails to measure up to some standard of perfection, what it “should be”.
If there’s something that is most foundational, the source of all things, then there’s nothing more foundational (either internal or external to it) that could limit it or make it not what it “should be” – it just is. Hence, if it exists, it’s unlimited and perfect. We could argue about how this relates to moral goodness, but if it’s not morally perfect, that wouldn’t be because it’s morally imperfect but because it transcends moral perfection and imperfection altogether. And it can’t “sin” since to “sin” is to miss the mark, to fail to measure up to some standard.
I should clarify, it’s not that you or I couldn’t personally distinguish two “sources of all things” but that two such sources would be indistinguishable in principle. Again, there would be nothing to limit any “source of all things”, so it would possess all perfections in an unlimited way. ‘Two’ such beings would thus be exactly the same with nothing to distinguish them. If they differed, then one would have some perfection that the other didn’t, which means that only one would be truly unlimited and most foundational.