Actually, this is probably true. 8 hours a day, averaging 18 miles per day for 486 miles over 29 days is not that much. I don't see where they say they had 20kg packs, which is quite high weight for a thru-hiker. (EDIT: See now where he says he had 40 pound pack, which is probably at heaviest point with full load of food, still high for thru-hiking and maybe a reason they logged less miles/day.) 18 miles/day is probably right around average for thru-hikers, who are mostly out for enjoyment and sense of accomplishment, not for speed. You have to be in decent shape, but it's not a high-intensity endeavor.
There is a fairly sizeable subset of thru-hikers who do go for speed, though. Many average well over 40 miles per day for several months and over 2000 miles. (They're definitely not carrying 40 pound packs.) They accomplish this mostly by hiking for longer hours each day, not by moving faster, since going faster breaks their bodies down. E.g., see this article:
http://www.adventuresnw.com/heather-anish-anderson-speed-que...