I think it's definitely feasible that she's not doping. She did a podcast w/ Carrie Tollefson recently where she talks about her history and build up with the sport. Worth a listen if you're wondering- training bit goes from about 4 to 10 min below.
She talks about how by senior year of high school she was training as a sprinter and only running 20 mpw. In college she built up 10 mpw / year- 30, 40, 50 and then 60 mpw. When she transitioned to being pro- she kept her mileage steady due to the increase in intensity of training w/ Jerry's group. She's now up to 80 mpw. Scary to think she could add even a bit more in mileage and/or reap the benefits of consistent training at 80 and what that could allow her to do in the 5k. Her coming onto the scene really started in 2016 in the 5k. She was probably ready to run faster the past two years in the 15 but Jerry thought she'd have a better chance in the 5k due to how stacked the 15 was at the time - Jenny, Shannon, Kate, Brenda, etc.
So no, I don't think it's an immediate conculsion that she's doping. It's possible, but I'd like to believe she isnt.