6 Pieces: Rested experience consumed is reduced by an additional 30%.Defeating enemies extends this effect, up to an additional 2 minutes.
Its also possible they bought a wow token and then went and bought all the heirlooms to get the mount. You see lower level toons with it because they probably have lots of heirloom pieces for their alts. Instead of each piece of gear giving a % experience bonus, Heirloom gear now has a set bonus that increases with more gear you have equipped. You have to purchase 30 heirloom pieces to get the bike. Heirloom gear scales with the player's level making it powerful no matter the level, and saving players needing to replace gear every few levels.
Heirlooms have been a part of World of Warcraft since The Wrath of the Lich King expansion in 2008, these unique items have gone through some changes over the years of different expansions and again were changed to fit the new leveling system added with the release of the Shadowlands.