Its incomparable cause that estimation has to be made for safe, so moderately challenging weather conditions. With rougher, the estimated time of the remaining part of hike prolongs significantly.
With starting point at Lucknerhaus parking lot.
2-3 hours to Stüdlhütte.
2-3 hours to start of Stüdlgrat route.
Climbing from the start of the Stüdlgrat route (usually from the Stüdlhütte) to the Frühstücksplatzerl takes about 4-6 hours for experienced climbers, as it's roughly 1,000 meters (3,300 ft) of elevation gain on the ridge itself, but total time depends on fitness, conditions, and route pace...
So starting from 7:00 AM
Would be approx 11:00 AM - 1:00 PM to get to Frühstücksplatz
These are NOT winter time estimations, and they did it with not much of a delay.
But that leaves not even full 4 hours before sundown. They had bit more than three.
Best possible scenario, Frühstücksplatz to the summit is 3 hours. Can go up to 5 hours, and were talking highely experienced and fit climbers.
So even if they had nice April weather, they would at the very best made it to the summit right before sundown and had to descend in dark.
Normal route descend takes another three hours at least. HAD to be expected much longer in Winter.