Let's not trivialize real totalitarianism in our haste to condemn stupid airport security theatre.
If DHS is targeting people when they travel just because they work on information security, that's pretty chilling.
Guard: "What do you do for a living?" Dev: "Software developer on a secure chat app"
Dev later claims he was interrogated about his work.
In regards to the specific question about the algorithm and the interrogator knowing something about computers, isn't it possible that he was personally interested in and asked while he had the chance to talk to someone who was familiar with that stuff?
If he had computer science experience he may have had a legitimate interest in it. That said, he may not have been a programmer, thus the open source part eluding him.
If it was the government they could just...look at the source.