Combined with the factoid it features under "how is this calculated":

    However, most native speakers have an active vocabulary between 15,000 and 35,000 words.

That tracks. Active vocabulary means the set of words that someone knows well enough to actually use in their speech or writing.

That's always going to be smaller than the set of words for which a person can choose the correct definition out of four options.

For sure there is a bit of selection bias with hackernews users. Not saying we are all geniuses, but I strongly believe we are, at least, more educated than your average Joe

There are words that I know from this quiz that I would never use in real life or in my writings. I’m not sure why. That’s the active vocabulary distinction.

You are almost always going to find people with above average reading and writing skills on an online forum - especially one with such "curated" audience and spartan UI.