The exchange was on the communication method of their choosing, right? As long as they accessed Discord via secure methods, there is no need for trust in anything but your own aptitude.
We've recently seen the police over here raiding the homes and offices of prominent journalists (quite publicly) to gain access to their sources and related material. :(
That's one data point in opposition (although not exactly revealing a 'source', the difference is splitting hairs).
Blogged under his full name until a couple years ago, blogs under first and middle still, full name still all over the internet and discoverable within a couple minutes, details of life all over blog still. Hell you probably don't even need to know his last name to find him. Scott Alexander Bay Area Psychiatrist is probably enough to go trawling through psychiatrist practice staff pages and find him fairly quickly.
Dude's basically wishing he was anonymous and living in denial because to accept that he isn't would be to accept that he really shouldn't continue his career and blog.
I think it's entirely possible for a person to both be willing to publish his last name and also go to prison in order to protect a source, all while remaining consistent in their principles.
Really?
Now, whether or not given journalists are following good enough data handling practises (eg encryption, etc) to still maintain confidentially is unknown.
We're at the point where the NYT is doxxing random bloggers. I don't think this is a good time to be relying on journalistic integrity for your safety.