iPad Safari scroll bar missing on some webpages
This is not a bug, but a design fault on Apple’s part. 

Going to the page you posted, the scroll bar on my own iPad is also not visible, but it is there. Scroll to the top of the page where the dark grey header is and you will see the scroll bar appear.

Visiting this post on my iPad, the scroll bar is visual, and you should be able to see it yourself.

Both are white pages, but the scroll bar is behaving differently, why? 

Apple predetermines the color of the scroll bar as the page is loading up, and doesn’t respond actively to what is displayed on the page (here lies the design fault). Until Apple changes how the scroll bar responds to content displayed live, this will continue to be an issue.

If you want to see this in action and if you have an iPad that displays the bar at the bottom of the screen where you swipe up to go home. One page that shows this issue exceptionally well is TheVerge.com. Visit this page and slowly scroll to the bottom paying attention to the “Home Bar” at the bottom and how it reacts to light and dark backgrounds and then the scroll bar, and it’s lack of any response, even disappearing when you reach the white section of the page.