We're using the new Word.Document.compare() API, which is currently in preview (https://learn.microsoft.com/en-us/javascript/api/word/word.document?view=word-js-preview#word-word-document-compare-member(1)) . It seems it is not possible to specify what is the original and what is the revised document. It seems to assume the current document is the original, and the target document is the revised document
See also this Github issue: https://github.com/OfficeDev/office-js/issues/4766