The test requirements for flocking bird encounters is performance based and requires that the engine be able to provide enough power to safely land the aircraft.
You can pretty much make a shambles of the fan (scrap every blade) and still make enough power to safely land.
the key is the fan blade tip clearance and what happens to that
The certification requirements have increased since the engines powering the B767 were designed.
Still there has to be a degree of avoidance coupled with even today's stronger engines else you can break anything that man makes, including the aircraft. With the latest generation engines the aircraft is the weakest link in a large bird encounter once in the air.