That's Reveal.js / Slides.com format. It became very popular in 2010s. The idea behind the 2-d navigation is that you can use left-to-right to move between chapters, and move down to dive into a specific chapter. This allows you to skip chapters due to time constraints. Or hide gnarly details about something so that these specific slides do not break the flow of presentation but still having them available for the audience online. Or, having slides announcing demos, but if demos do not work the down slide would have a video demonstrating how the demo is supposed to work. Many possibilities like this. Also the slides are produces using Markdown, so the format was appealing to many authors.

However, doing chapters well turned out to be tricky. Ideally you want them to be of similar size and have 3 to 7 of them in the talk, but many presentations aren't structured like this. The rise of Slideshare and SpeakerDeck for sharing slides in mid 2010s caused this 2-d navigation to go out of favor: those services only support linear static slides. This is also a reason why people use fewer animations in slides nowadays and why tools like Prezi didn't catch on (that was another presentation tool with non-standard navigation that went out of favor very quickly).

Many people still use Reveal.js to make their slides but they stick to left-to-right nav only.

Going to the link and just hitting the spacebar worked for me. Next slide, and so on. Firefox/Linux.

I have the same problem with the mouse (little page marker overlay covers the down arrow).

But using keyboard arrow keys work for me.