That's your problem. iOS Webkit has buggy scrolling behavior if you use fixed position elements or iframes. Unfortunately, Apple won't allow non-buggy browsers on their platform, so there's nothing you can do about it.
What you are describing was a problem with inconsistent scrolling behaviour, and it was fixed. On this site scrolling is just completely broken and it's dev's fault.
Interesting. Because with Firefox 59 for Android, both kinetic and inertial scrolling work as expected (just tested). Odd that it does not work for iOS Safari.
I don't know about this specific case, but mobile Safari versions have a history of doing special things like CSS position: fixed works entirely different on that platform.
Please consider using default scrolling