In a word, ././ is redundant. It's a hack to convert absolute paths (e.g., /usr/bin/foo) to relative paths (e.g., ././/bin/foo) without changing the length of the string. Binaries have internal addresses, so if the length of a string inside a binary is changed when patching the binary to make it portable, the binary will no longer work because things will no longer be at the expected addresses.

bruno@thinkpad:~$ cd /bin
bruno@thinkpad:/bin$ ././echo test
test
bruno@thinkpad:/bin$ 

But for the life of me I can't figure out why someone wouldn't simply use plain ol' ./. Maybe the AppImage guys are unaware that the simpler form is equivalent? Maybe ././ also works in other OSes but ./ only works in GNU/Linux? I'm at a loss.

If none of my Devuan friends know, I may contact the AppImage guys to ask them.

Have you ever encountered ././ before? I have only ever seen it in the AppImage guide in the link above. Please, what's the difference between ./ and ././? Internet searches have not turned up anything even remotely relevant this question.