I think the recommendation against riding it solo has to do with the remoteness of some of the route sections and the ability to get help if there's an issue. If you're traveling with another person, and if you're incapacitated for some reason, you have someone with you who can call for help and potentially stabilize you if you are seriously hurt. If your bike breaks down, you could be walking for a period of time before you get to a spot where you could either get repairs or hitch a ride. Cell phone coverage is spotty in many areas, so I wouldn't rely on that as an "out". I rode south from Rooseville to Ovando, and then back north to Swan Lake with two friends over 6 days this past July. There were at least two days where we didn't see a single vehicle other than at camp sites.
All that being said, we did encounter a number of through riders, both male and female running solo on the route this summer. If you take care of yourself and equipment, you should be fine. I would consider investing in a spot tracker if I was riding solo, just so friends and/or family would know where I am, and could notice if I haven't moved for some amount of time.