For me the scrollbar disappears soon after the pointer is moved away from the code area. To read the overflowing code, you can scroll horizontally, and then move the move the mouse out of the way. In a second or so, the scrollbar disappears.
For context, macOS has three options for scrollbars:
I tested the link in the OP with the other two options, and “When scrolling” behaves the same way (i.e. scrollbar fades out once mouse is away from the code block) whereas “Always” will display the scrollbar below the one-liner in your linked example:
