There is pretty severe rate-limiting. I've never tried to figure out the exact rates, and it's possible they vary per-user based on some opaque historical factors, but whatever they are, they're low. Try to post 40 articles in a row and it won't let you. Combine that with the fact that new accounts can't submit anything, and that takes care of a lot of low-hanging fruit with respect to spam prevention.

But it isn't 100% spam free. It's just free of the really obvious things usually plaguing blogs with no filters on comments offering Viagra and work-from-home MLM schemes. But a whole lot of content is submitted that pretends to be more or less objective analysis of some tech problem but is really a thinly-veiled ad for a service solving that problem, or criticism of something posted by a competitor. And CEOs/founders are constantly out there shilling for their own crap in comments as answers to user queries. That kind of thing tends to get downvoted and/or flagged quickly, but it's still there.