The Boeing 777-300ER has the following weight specifications:
Maximum Takeoff Weight (MTOW): 775,000 lbs (351,534 kg)
Maximum Landing Weight (MLW): 554,000 lbs (251,290 kg)
Comparison:
The MTOW is significantly higher than the MLW because an aircraft takes off with a full fuel load and payload but needs to be lighter for landing due to structural limitations.
The difference between MTOW and MLW is 221,000 lbs (100,244 kg), which means that if a 777-300ER needs to return to the airport shortly after takeoff, it may have to dump or burn fuel to reduce weight before landing safely.
It is therefore pretty obvious why the aiircraft had to dump fuel.
To reduce from MTOW to MLW, a Boeing 777-300ER would need to dump 221,000 lbs of fuel, which would take approximately 39 minutes at the standard fuel dump rate.
That figure is to reach MLW, and assumes that the aircraft's front landing assembly could withstand the weight involved. If there was any doubt about that fact, it would explain why flying a holding pattern for longer would be a safer option. When lives are at risk, it is neccessary to apply precautionary principles.