It seems obvious to me that she did this as part of her rehab, trail running is lower impact than pounding pavement, she's been doing inclines on the treadmill so probably figured some hills would be fine. She clearly took it easy, as Sage has confirmed, its not an overly technical course so if she was going hard she would've won. If she'd been out for a long hike would anyone care? But because she wanted to have some fun, throw on a bib and soak up the atmosphere of a trail race its suddenly a problem.
As for using a different name, if she's using it as rehab she probably didnt want to draw attention or expectation. iRunFar would have done a whole article on what to expect from an Olympian taking on Speedgoat had they seen her name on the start list. She'd have been surrounded pre/post race and with the expectation to perform she probably would have have pushed harder and compromised her recovery. Also, as a pro-runner, she'd have this finish against her name, which if she's not actually trying to win, I can see why you'd want to avoid having that on your resume.