It triggers half of HN because taken at extreme (abusing) this advice will render code unreadable again.
It is never clear cut. But if you never reuse the same code, or it is targeting the wrong abstraction layer (you can't reuse the function without introducing another set of arguments) and you are merely encapsulating two already very readable lines of code, you might end up with a too much redirection and an increase in mental load.
What I'm trying to say is: It might be even harder for the outsider to follow/read the code, just simpler on first glance.
If the lines of code were already very readable, they wouldn't require a comment. Comments are for interfaces or complex or inobvious code that can't easily be made more obvious simply through an expression of code IMO.
The code example of the OP is excellent because there are literally so many more avenues for code readability that are all superior to comments. First of all, the same code can literally be reused in the next line:
addScrollbar(HORIZONTAL);
addScrollbar(VERTICAL);
Do you frequently or always need to add both scrollbars to the UI? Another chance for reducing repetition and error-proneness when refactoring:
addScrollbars();
There's no increase in mental load because chances are you'll never need to navigate into these functions to know what they are doing at all, you can read over them as you would normally when you read the "This adds a horizontal scrollbar" comment, and trust that the implementation does what it says on the tin.
It seems you did not try to understand my comment:
I said "taken to the extreme". Put differently: If you are going to wrap every pair of lines into separate functions as a way to structure your code, you code will end up becoming functions calling functions calling functions - and whenever you try to read the code, you are required to jump through all these redirections.
Now instead of a bunch of prepared and labeled dishes you have an API border in the middle of your kitchen, which of course will greatly help at slight changes in the menu. When in doubt, just pepper it with more enums and boolean flags. /s
This is literally always the right answer. Create a self-explanatory function or just use readable variable names.
Or even createScrollbar(Direction.VERTICAL) if you hate repeating yourself.