No they don't. They use their own adserver (bought from adzerk) to physically serve the ads, but they have always come through RTB connections to ad exchanges.

There are private marketplaces and "automated guaranteed" deals to isolate their inventory in its representation and pricing from the rest of the market but the actual campaigns they get exposure to, and the creatives delivered, aren't special to them.

I've seen plenty of SO-specific ad campaigns being run on the site.

They run house-ads for their own jobs board and products, and they will have private marketplaces inside the exchanges, but those campaigns and creatives will still go through the standard adtech supply chain with all the JS-based layers added on.

https://www.stackoverflowbusiness.com/advertising

They do sell job postings and have sponsored tags so it's not all network ad revenue, but that's a minority of the income. Since they released their Q/A SaaS product now, maybe they’ll shift to selling that as the primary revenue stream.