What about this?

    function v(x) {
     return
       x;
    }
    
    v("yeah");

returns undefined

What about it? Yes, it returns undefined. Has nothing to do with semicolons. You can add semicolon to every line of your example, or remove each and every one and still it returns undefined.

He's putting it in comparison to the previous example https://news.ycombinator.com/item?id=9718410

Newline matters / doesn't matter, the same with the semicolons.