Are Midoriya and other quirkless people actually quirkless?
I’ll wait for the manga to comment on Midoriya, but the other major “quirkless” character we are shown is the original One for All. He was All for One’s brother and thought to have no quirk. However, we soon find out that his quirk was one with no effect - the ability to transfer quirks. It requires the action of All for One forcibly transferring a quirk to him to manifest itself.

I believe that other quirkless people have similar quirks that don’t manifest because they don’t have the right stimuli.