The purpose of a box squat is to develop power out of the hole, out of the bottom position. This is done by eliminating the stretch-shortening cycle. There are other ways to do this such as pin squats/rack squats. Westside Barbell popularized box squats maybe a decade ago in the powerlifting community.
My question here is why? She's doing a box squat with ankle flexion. If the weight is challenging as a squat then why the ankle flexion? If instead it's more along the lines of a power exercise with a lower weight then why the box squat? Why not just do regular squats with a lower weight and added ankle flexion.....Almost like a jump squat.
Like any squat variation to be safe and effective you must remain tight. This is lost on most clients. You must remain painfully tight to ensure the bar travels along the right path. With a box squat it's even moreso as there is a tendency to relax when you sit down.
A distance runner just doesn't need much complexity when it comes to strength training. It could take you 4-5yrs to really get competent in the squat.
Alan