Their last album was the #1 best-selling album on the Billboard R&B/Hip Hop chart and they sold out every venue on their tour. I think they're reasonably well-known.
According to https://www.billboard.com/music/run-the-jewels, RTJ 3 peaked at #13 on Billboard 200. That's good, but it isn't being 'practically the biggest name in hip hop' good. Plus, most people probably wouldn't recognize Killer Mike or El-P (although more people may recognize Mike over El-P from his time supporting Sanders).