El-P is not nearly the biggest name in hip hop. Maybe in the online world him and RTJ are big, but amongst the general population of hip hop listeners I don’t think he’s very big.
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.
It may have been best selling when initially released, but that relies on a lot of factors (other albums released at the time being a major one).
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).