Yes, it uses calibre via its command line utility, ebook-convert. It does add a lot of classes, but I have learned to live with that!
The resulting epub is attached for your perusal. I also attach another version that has been tweaked/fixed i.e. removed larger images that were hyperlinked to (this works well when reading on a computer monitor, but not so when reading on a 6" device) and improved the toc.ncx.
I find that calibre can properly convert most .html and images ebooks, but it may be necessary to "tweak" that epub after conversion. This is usually to "fix" some use of the CSS classes (i.e remove page-break in unwanted places/classes or add center to a tag).
I also use ePubFixer (found in our ePub formats forum) to clean up the calibre generated TOC entries (located in the toc.ncx) as I have done below with my conversion of PG Etext #6317.
All in all, the conversion took about 1 to 2 minutes and then another 10 to 20 minutes to tweak things to my (and most people's) liking...
The beauty of GuteBook is that YOU get to download and customize your own version of that PG book, just the way YOU like it (after some brute-force fixing/tweaking afterward).
Enjoy!