A aluminum extrusion is made different than steel tube. completely different animal with how it's made as well as the tolerance it has. He will be fine with flatness as long as he got his mounting surface level and on plane. It is interesting that it's so hard to find the flatness tolerance for 80/20 extrusion though. But regardless I guarantee it's way flatter than the 1018 tube the Saturn is made of.
If I personally built a extrusion machine I would buy pre machined lengths for the surfaces that need to be flat (gantry and rail mounts). I don't remember off hand what the company name is.
But like I said I'm sure he'll be fine with his table he's being very thorough with everything. And if our tables are turning out good parts his definitely will too.
Dan