That should of course have been https://gitlab.com/lilypond/lilypond/-/blob/master/ly/lilypond-book-preamble.ly for the authoritive repository but for some reason Google is not listing the gitlab direct link and showing the github mirror direct link as the first hit when searching for 'lilypond-book-preamble.ly gitlab'. For gitlab it only lists an issue.
This apparently interfered with -dcrop='#t'. No it works both from as a cli argument and as set in the file: #(ly:set-option 'crop #t)