They basically say that an Avro aircraft went airborne before that landing, but had not yet overflown the localizer antenna by the time the B777 touched down.
The seconds the Avro approached the antenna are the moment the B777 veered of track and eventually left the runway.
A go around was attempted, but "TOGA had no effect" (already on ground), the autopilot (roll out) was disconnected about the time the plane hit the grass.