I just did it by:
- downloading the zip version of the document and unzipping it to a temp folder
- adding cover.html in that folder to calibre
- starting a convert to epub of the ebook resulting from #2
- setting table of contents level 1 detection to //h:h1
- letting the convert run
I got a nice epub with a level 1 toc. If you also want level two, put //h:h2 into the table of contents level 2 detection box.