We don't really know what questions he asked. For all we know, the developer has a bone to pick and is grossly exaggerating his claims.
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?
While I'm generally the tin hat wearer in my circles, it's perfectly logical that he was just curious.
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.
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?