Nice find!
I wonder when it was put up, it took me a while to track the original down
License-wise on the site it says: > You can use it for anything you want
Which implies “public domain”, and …
https://github.com/opless/small-prolog-walnut-creek-original
And on the copy on the Walnut Creek CD it explicitly says;
These diskettes contain a public domain Prolog interpreter called Small Prolog version 2. In the USA the C User's Journal will have the latest releases.
It has been compiled on an PC clone, on UNIX and on an Atari ST.