Don't forget that Zion is not a typical lineman. He came in around 240 or 250. (As an aside, I saw him on campus when he had just arrived and I thought he was an oversized linebacker.) He's probably leaner than most lineman. I suspect that he's likely above 13% b/c he's likely had to eat plenty to get up to the 312 he's listed at, but it's not entirely impossible either.
(If any of you remember Darryl Gardener on the Fins, he was massive at 290 or 300+ and likely under 10%. Granted, he was likely on stuff but it's possible. IIRC, he became a professional bodybuilder after his playing days ended.)