In this particular timeline, we see that you get a tease of the timeline elements towards the end and then you get a down arrow, that you have to click on to load more of the timeline content.
If you are having trouble with the pen, try the archived copy on GitHub
TimeLine Tease Snippet
See the Pen Advanced TimeLine : HTML / CSS / JavaScript by MO7AMED (@mo7hamed) on CodePen.0