In UX terms, the page above the fold doesn't afford scrolling. With a blank white background and nothing to help the eyes explore, it can take a bit to notice the scrollbar, especially if one's browser/OS doesn't have an obvious scrollbar style.
Any scrollbar would be nice, but I'd like to introduce you to mobile browsers that hide scrollbars by default.
Most desktops do that too, these days, and only display them on hover.
Sadly, Firefox doesn't show the scrollbar by default. Stupid decision imo, but here we are.
What? That makes no sense -- scrollbars are there so you can drag them around and scroll, not just to indicate scrollable content. Not at all analogous in this case. The site looks nice overall, but the explicit indicator is bad design regardless.
Historically, at least, that is exactly what the scrollbar indicates, which is why it is either not shown at all, or shown disabled (i.e. visually distinct) if content is not scrollable.