I've heard people say this before, as if there's no way to use anything but your permanent dock apps in split screen. But if it's not in the Dock, you launch it, and now it's in the dock. That's why the recent items section on the right side exists.
My personal gripe with iOS multitasking is that it's easy to make a second instance of an app by dragging and dropping (such as Safari tabs to split screen), but then it's not as easy to close the second copy when you're done with it. When split screen was a special implementation within Safari, dragging the last tab from one side to the other would automatically collapse that side and bring the remaining one back to full screen. Now it leaves an empty Safari window hanging around.
If you want to close that, you have to make one side or the other full screen, open the app switcher, and swipe the empty one up off the top.