I've got it working. The add-on does exactly what it says on the tin: creates widgets. But it might not be obvious what a ‘widget’ is in the Android world…
Go to the home screen. Find an empty space, and tap and hold on it. The screen will zoom out a bit, leaving room for three new icons at the bottom: WALLPAPERS, WIDGETS, and SETTINGS. Tap on WIDGETS, scroll down to the Termux section, drag the ‘Single shortcut 1 x 1’ onto your home screen somewhere, and then choose from the list of shortcuts (i.e. the files you created in ~/.shortcuts). Hey presto!
(Caused me some head-scratching and web searching, too…)
It looks like any files in ~/.shortcuts are eligible; they don't need to be have the executable bit set, or have a particular filename or extension, or have anything special on the first line.