I disagree with Socrates that he knew nothing in a literal way. What I believe he meant was, that he is prepared to open himself to new knowledge, that may debunk his current knowledge of the ways of the world.
It is, logically, impossible to know nothing. The fact that we have at least one sense at out disposal, grant us the inevitable “fate” to know at least one thing. Our senses are, after all, the initial bringers of information, that even the wombs we grew in, we cannot escape from receiving sensory knowledge, thus ending our pure ignorance, if there was ever such a thing, to begin with.
The expression “I know nothing” can easily be debunked - it is sufficient to know at least one thing, in order for that expression to be incorrect.
Therefore, I am convinced that Socrates meant that, he is ready to find his current knowledge to be incorrect, by any other knowledge he may receive from the External World.
In other words, by saying “I know nothing”, Socrates might have meant that his mind is open to new ideas, that may be truer than the current ideas he possesses.
Being symbolically the founding father of Western philosophy - Socrates wouldn’t be incorrect on purpose; a figure that has put his life under danger in the name of finding truths, wouldn’t mislead us, nor himself, on purpose, because misleading keeps one away from the truth, and why would a truth-seeker purposefully keep themselves away from which they seek?
I am thus confident that Socrates didn’t say what he say literarily. “Knowing nothing” in this case might be an indirect indication to his preparation to know more, and to put the current knowledge he has, in the test of evidence.
It isn’t that “knowing nothing” is a sign of wisdom; it is one’s readiness to accept the truth, instead of finding ways to “hide” from it - in the form of denial, of ignoring-on-purpose, and so forth.
What we may receive as “knowledge”, may be nothing more than a mistake that hinder us away from realizing potential truths. By saying “I know nothing”, Socrates perhaps meant that he doesn’t hold anything as “knowledge” until he will receive proof that either confirms or denies what one sees as “knowledge”, as actual knowledge.