FWIW, in the second 25km, I was pretty stuffed. A very hard few weeks leading up to the race for me meant I was just aiming to finish. I had no interest in beating times, getting a decent place ... anything like that. Having said that, I was surrounded by other pretty tired people most of this second half of the race, but the difference I noticed was that a lot of these people were not skilled riders. I didn't have to dismount for a single obstacle in the second 25km, and I would guesstimate I passed nearly 80-100 stationary people on said obstacles (probably 30 in grass trees, another 30 in Yo Yo and the roots following it, and the rest throughout the last bit of singletrack among the grass trees before you pop out onto the fire road).
Just before Yo Yo there was a big water tank where about 50 riders were standing around when I went through without stopping. I am not sure, but I think some were 100km riders are well? But still ... a lot of people passed in the space of 5 seconds.
Plus in the last 3 or so km, I got a burst of pseudo energy once I knew the end was 'in sight' and I think I passed about 15 people from here (about 10 of which were off their bikes in that last technical pinch before the oval).
All up I would have passed quite a few people on that second 25km. Most of them would have passed me earlier in the day on the steep climb up from Lake Elizabeth, where my single 34T chainring was killing me, or half way down Red Carpet, where my friend's shoe wouldn't come off of his pedal, and we sepnt 10min trying to get it out (I reckon about 50 or so riders passed us here). Or the 10 min I spent at the oval waiting for him to get this fixed. A lot may have passed me later on as I walked up In Quads We Trust, and the entire climb on the other side of the road after this. But I don't seem to remember many riders passing me here ... lots of hot, defeated looking people, walking like myself.
I haven't looked at stats or times. I have no interest. I just know I rode the entire 50km course (which I too think must've been longer with the addition of grass trees and I'm sure an extra climb on Yo Yo that was skipped last year). I know I passed a lot of people in that second 25km simply because they were off their bikes during technical parts, and never seemed to catch up to me again.
If this lady was simply able to ride the obstacles without stopping, and fit enough to ride more of the climbs than me, then I'm sure it's possible for her to have done what she did.