The ‘URL’ has always been the full URL. The fact one can type an incomplete URL like ‘google.com’ or ‘www.google.com’ into a web browser and it be interpreted as ‘http://www.google.com’ is, I understand, something built into the web browser. (Indeed, I see the forum software is automatically doing similar with the first two even though I added no mark-up and don’t want [sic] a web link there and can’t remove it!).
So, is the Create Web Link dialog faulty no. Could it be easier for those unfamiliar with URLs? Tinderbox by habit doesn’t throw lots of effort so a partial URL placed in the $URL field will give a silent fail. I don’t know if the newer macOS frameworks used since v6 allow the same sort of fixing of user input as done by web browsers, but I guess that would be a feature request at this point if you want to make it. As users, we can’t modify the current behaviour.