To be fair to the DM (reluctantly) the story doesn't say "passenger runs across runway" – the word "runway" is only in the web page title.
I wonder what was going through the pasenger's mind, such as it is. Did he think that if he caught up with the aircraft, they'd stop and let him on? Really? Though the story says it was a 737, and some 737s do have internal steps that deploy from under the front left door, so if this aircraft had one, they could have let him on, but I'm sure they wouldn't!
By the way, do any larger Boeings or Airbuses carry their own steps?