Earlier quoted context omitted.
Arrow keys also work.
That said, this is pretty poorly designed. I also had no clue what to do for the first few seconds after I clicked the link. It was only at slide 38/41 that I noticed the tiny, hidden controls on the very bottom blending into my status bar.
I didn't even notice those. Fortunately, I've worked with systems like S5 slideshows, and this looked a lot like a slideshow (no scroll bar, large text, numeric fragment in the URL).
Usability could definitely be better, but I will admit to writing up similarly boring slides in the past.