Viator doesn't operate any tours at all. It is just a re-seller of other companies' tours. You can figure out the name of the company actually operating the tour by scrolling down on the listing page and clicking on "Additional Info". You should find the name of the company there. If you book directly with that company, either the company or you--or both--will come out ahead financially, because Viator takes a very large cut when you book through it.
I would certainly go to Naples on my own (as I suspect you're planning to do), taking advantage of the very fast rail service. Pick up a tour in Naples. I haven't taken any, so I cannot offer a personal recommendation.
The Mondo/Shared Tours folks have a three-hour walking tour of Naples: https://www.sharedtours.com/. Scroll down a bit to see the Naples tour. The tours need at least six participants to run, so this may not work out for you. I don't know whether you can tell from the website how many folks have signed up.