They meant if you close it and, say, want to read or follow along with a book in iBooks and not use split view, you have to multitask back to Prompt every 3 minutes or iOS will suspend the app, killing your SSH connection in the process. Since Prompt is closed source and on the App Store, they can't use mosh which solves this problem.
iOS only allows background connections to be maintained for three minutes. That means if you keep the app in the background longer than that, for example while checking Safari, the connection breaks. Every app has this limitation (TeamViewer, etc...) and it is usually solved by showing a notification to make the user switch to the app again. It is described on Prompt's KB too.
Blink also has this problem, but Mosh is able to recover gracefully.
You might provide a video clip of a demo to help make a purchase decision.