The original tunes were all in a file called Captain_Kidd_abc.html, which no longer exists on the site. Whoever put that file there probably moved on, and their files were deleted.
An interesting aspect to this case is that (as of early 2021), the site contains some song collections, and the web pages have some plain-text notation that shares some features with ABC notation. It isn't real ABC, but there may be some vague connection at work. So I haven't put the site on my "Bad Sites" list, in case some more readable (by software) music happens to appear there again.