Legendary for PCs is described as 500 karma. Legendary for NPCs isn't, it's just described as "being in this book," basically. The movers and shakers in Shadowrun, the folks with the high rep? Some of them are Lofwyr, or Lugh Surehand, or Martin DeVries. Some of them are powerhouses, and their stats reflect that.
Some of them, on the other hand, are just folks who were in the right place at the right time, and have built a rep for themselves by being recognizable names -- the NPCs who chatter with one another about new gear, spread rumors about upcoming jobs, and upload data files to Jackpoint for the rest of them to pick at like vultures. If you're a recognizable, memorable, Shadowrun NPC, you had a shot at being in Street Legends. Maybe you were in a couple novels (Kellan Colt), maybe you're a regular Jackpointer, maybe you're a long-time lurker like Bull who's been used as an NPC a few times before...there's no "500 karma" requirement to be an NPC in Street Legends and get the (optional) rule that lets the campaign kick in the cinematic rules. Their version of the rule is there to reflect that a household name is on the scene, and everyone in the game suddenly kicks it up a notch.
As a wholly separate entity from that rule, there is a "Legendary" Quality that PCs can buy. This Quality comes with karma requirements (and GM's consent, again), and allows that single PC and only that single PC to score a hit on a 4+ instead of a 5+.
People keep crossing the wires of those two rules, and I really rather wish they'd stop.