Here is how I see it. At best, you interrupt a candidate in the middle of a technical interview to make a joke. At worst, you ruin the interview experience for the candidate. To me, the cost/benefit is clear cut.
During a technical interview, you are evaluating the candidate’s technical knowledge. You are not there to become friends. There will be time for that after the candidate goes through the interview process.
The only other choices for the interviewer are to bluntly say "you missed using semicolons" or to not say anything and be left to wonder if the person knows how to really write in C. The author of this post seems to think even considering any other choice than the last is rude since "whiteboard code doesn't compile so semicolons are syntactic sugar".
But if it does matter, baking in some understanding of the charitable reasons why (that the interviewee is proficient at multiple languages, especially in the popular ones at Apple like Swift) is actually the nicest thing the interviewer could have done.
The post author seems to me hostile to being corrected. Given that every code review I've been in requires being corrected dozens of times about errors large and small, I wouldn't want to have to deal with their uncharitable attitude towards feedback from coworkers/collaborators.
Code reviews after already being hired and accepted into a company are not comparable to interviews during the application process, when the specter of rejection still looms.
Anyhow, to answer you, on both sides of interviews, I've experienced bad chemistry early in the session. Could be any number of factors, even just one of the two of us getting out on the wrong side of bed that morning. I sometimes will make a joke to lighten things up and get things back on track. Sometimes it helps, sometimes not, but I like to feel over time I am learning better how to interact with humans, and improving my odds, leading to more positive interview experiences and less wasted time. Things are never "strictly technical" with humans.
(all that said, to be clear, I have never brought up the candidate's sense of humor in providing feedback, although I don't see a problem doing so if humor is in the team's values manifesto or the job description, and we've agreed on a legitimate way to screen for it (seems hard!). Curious to learn others' thoughts on this subject!)
1) felt that the tone of the interviewer was negative
2) was in the middle of writing their solution
3) did not actively ask for feedback on syntax
Point #1 is subjective, but an interviewer should err on the side of caution when making curt remarks like this. Taken together, points #2 and #3 are important too: as an interviewer, you should know when (or even whether or not) to provide feedback.
As for why I believe the comment above phrased it better:
1) the interviewer is clearly acknowledging that it can be challenging to use a different language in an interview
2) the interviewer is clearly stating that the syntax mistakes being made are minor and nothing to worry about
Personally, I would still avoid making this kind of comment unless prompted by the candidate - e.g., in reply to "I think I might be making a few mistakes with syntax here". I would also avoid providing any feedback.
I also agree that an interview is not purely technical: there are other _passive_ signals you can use to build a better picture. I don't see how you would be able to judge sense of humor reliably in an interview setting, though.
A negative reading is “you are screwing this up enough for me to comment, you need to justify why”
If it’s intended to be reassuring then phrasing it so it’s clearly not a criticism is better. If it’s intended as a criticism then phrasing it clearly is also better.
The way I can see it being a joke, as claimed, is insultingly. Like a dentist watching a trainee dentist doing a filling and saying “you must have been doing masonry work lately”. Haha but the subtext is “you’re really doing a bad job of this”.