On non-Keyman iOS, a longpress pulls up the list, and moving horizontally (not necessarily vertically) moves the cursor above your finger. This is Press+Right+Release. The finger moves along at the original key latitude and doesn't need to cover the choices.
On Keyman, even on a longpress with one alternate option, the motion is always Press+Right+Up+Release, and your finger must cover the target choice to select it. This is one step more complex. If only one longpress is shown, the process would be Press+Up+Release.
This is a feature request to extend the touch box on Android/iOS downward below the basekey so that the finger doesn't have to move up to select the popup option, only left/right. I actually achieved this through CSS for KMW, before finding dragons.