Earlier quoted context omitted.
It sounds like you might not be quite getting the concept behind this sort of interview. Although some code is written, this is (usually) a test of how you talk about code. You want to show that, if a co-worker asks for your advice on how to solve a tricky problem, you will be able to help them figure it out. Part of this is understanding what they're asking for and adapting. If someone wants to know how to document…
>It sounds like you might not be quite getting the concept behind this sort of interview. Although some code is written, this is (usually) a test of how you talk about code. Assuming everything the GP said is true, what you're saying is completely contradicted by the fact that they were dinged for not using a null coalescing operator.
You could thank them for pointing it out, say you're not sure you want to use it (if you don't think it's better) and then get the conversation back on track. And, hopefully they'll think you handled it smoothly and won't take off points, but you'll never know how important it is to them that you're fluent in Swift. Maybe not at all, it's just an aside?
Which is to say, usually you can't tell in an interview what they're really grading you on. It's nerve-racking, but you just have to try to seem reasonable and hope that works.