My reply was less about whether or not it's a good interview question and more about the comments people are making in this thread that the question is completely invalid because only a "fool" would write code like that.
In practice, unless you have some extremely pedantic, end to end testing, your first hint that something is going wrong is probably going to be malformed output. TypeScript won't catch this error because it doesn't do runtime checking, it trusts that type annotations are correct. It's very possible for code like this to pass through without causing any exceptions to throw. Indeed, JavaScript was originally designed to "keep on trucking" in this way, and that's why we have this degree of type coercion. So you're going to be working backwards from a result rather than forward from a source. If you already know how your code works and you see a weird result like that, knowing the type coercion gotchas can help you work backwards to understanding where the change occurred.
But in the former issue, I don't think the question on its own is bad for job interviews. How the question is used is what determines if it's good or bad. For example, it is a little unreasonable to expect people to know every single, little gotcha in JavaScript and the exact way in which they misbehave. But I do think it's reasonable to expect people to know that there are gotchas and that this is a common one. I'd accept as a valid answer, "I don't know the exact results, but I do know that this doesn't concatenate arrays and instead does something unexpected".