Moreover, there's more to the point Gladwell is making than that Snowden wasn't an insider. The core point of the piece is that Ellsberg's insider status enabled him to leak carefully, and to redact strategically. Snowden demonstrably did not do this: not only did things get published that weren't in the American public interest, but other things that were (for instance: traces of the Juniper/Netscreen backdoor, or implants in other VPN products) were never published.
It's not just that Snowden wasn't an insider, but that he lacked the ability to leak carefully and strategically --- and so the public outcome was inferior to the Pentagon Papers.