Engineering has large degrees of opinion mixed into any non-rudimentary implementation.
The result, even just the code itself, is also not judged on a binary yes/no scale, which again leaves a lot of room for opinions and bias to muddy the water.
I've done hundreds of engineering interviews. Conducting objective interviews is very hard. Anybody thinking they are objective either has never done interviews or is fooling themselves.
If a programming exercise is subjective, from what perspective is it subjective in bias of? Likely the author's, who's bias will be derived somewhat from their race, gender, sexuality, socio-economic class, background, peer group, etc.
Code is correct if it works. But plenty of "working" solutions will be rejected as being "incorrect" based on current trends, internalized philosophy, preferred approach, etc. etc.
As someone who hires engineers, it is difficult if not impossible to separate personal bias and preference from an evaluation.
But there's a billion ways to write that code. Which ones will the evaluator prefer and/or reject? That's where subjective opinion comes in.
You are judged and reviewed on a myriad different factors other than this. Are you a "team player?" Do you mentor others well? Do people like you? Were you picked to do complex projects this review cycle? Are you too loud? Too timid? Do you ask insightful questions? Is your coding style "normal"? Do you dress professionally? Can you be asked to interface with clients?
Every little thing that can be used will be used to form a composite picture of you as a person. It's wrong to expect a foreigner like me to be judged on whether I know baseball stats but I'm sure someone somewhere has made an inside reference about baseball around me and written me off when didn't even acknowledge it.
That's what diversity programs and affirmative action programs try to short circuit.