If you travel frequently across borders, you will often be taken aside and asked questions. It is mind-numbingly common, it has happened to me in Schiphol (EU) several times. Very annoying, but not at all the same as people being beaten and censored and put into gulags for mild civil forms of political dissent.
Let's not trivialize real totalitarianism in our haste to condemn stupid airport security theatre.
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.
I've been rather thoroughly questioned about just what exactly openbsd is and what it's good for and why I'd be silly enough to work on it for free just about every time I visit Canada. It's annoying, but doesn't feel nefarious. Maybe I should start blogging about so the Internet will throw me a pity party.
Let's not trivialize real totalitarianism in our haste to condemn stupid airport security theatre.