If the stoppie were done without any use of the rear brake, the back wheel would probably be spinning at the same speed as when it left the ground. As the front wheel slowed under braking, I would expect the ABS to note that the front wheel is slower than the back, suggesting it was starting to slip and therefore activate the front wheel ABS preventing a stoppie.
If you have any rear brake on, then the back wheel will slow rapidly as it lifts from the road. the ABS will see the BACK wheel slowing compared to the front and will activate the REAR abs, leaving the front alone to perform the stoppie.
you I could be talking b*llocks.