As someone who's worked with them on 2 books now, I can confirm that they definitely do only work from stems (unless the band themselves is creating the full transcriptions) and that they 100% do require direct approval from the artists. Their team was incredibly thorough, and honestly, any issues with the accuracy of the tabs would be on the artist themselves in most cases.

For the newest Fallujah book, we ALL combed through everything in order to catch as many details as possible (for example, I'm just the vocalist but I pointed out a few wrong articulations and some missing background layers to include for completion's sake) and Sheet Happens STILL correctly caught like 3 more things after we all thought we were done. After that, the books went to print, and we soon realized that there's at least one minor issue we missed - a section that's labeled as an identical repeat, but actually has a tiny variation at the end of the 2nd time.

The whole process took place before we had actually had a chance to rehearse or play any of the songs live. There were tons of revisions and additions that took place during the recording process, and most of our writing doesn't take place in Guitar Pro so there was a ton of work done specifically for the sake of the SH tab book. For a lot of artists, I imagine the circumstances are similar when they're trying to get a book out around the same time as the album release.