Crew can open the door from outside unless the pilot on the flight deck is actively preventing it from being opened.
The NYT statement is unconfirmed yet, speculation in fact, but whether it was a deliberate act or not, there will be new SOPs re the cockpit door and leaving one single pilot in the cockpit. Many airlines, and I think all US ones stipulate that if one pilot leaves the flight deck a cabin crew member must sit in the jump seat while gone. I can see a new job description combining, Steward/Security Officer/Third officer.
