I agree, my guess is it's caused by the lack of white space.
The margins are too narrow which makes it unclear what belongs to what and where the boundaries are. That can feel messy as it all sort of blends together visually. To compensate they used lots of borders, but those tend add more density without enough white space.
I don't think the actual layout is that messy, it's a nice column layout, seems decently structured, nice sections. It's just really hard to see atm.
I think if they use this exact layout, but just give everything some "breathing room" it would be much better.
The other thing is the images. They use screenshots of UI without making it into a graphic; meaning it doesn't look like a graphic, easy to fix, would improve things a lot.
The second thing that stands out to me is the text in the images. Beyond bad practice it's messy as now there are two titles that are competing for my attention. I think just the text on the graphic (as actual text) or just a graphic + text as it is now, would be much better!
Anyway, I agree, it's a bit messy. That said, I think it could be quite nice without too much effort.
I would love to play around with this, I think it has a lot of potential.
