Nigel and 20d : the Gillham box error that caused the incident you are ref to, has been identified and corrected in the meantime and is very unlikely to happen again , and definitively not with modern avionics , as the 2 a/c involved were brand new.
Even if one ACAS unit did not function , was u/s , not fitted, etc.. its SSR +mode C will be enough to activate the other unit and prevent the collision.
So the cause , if indeed there were a collision between those 2, lay probably elsewhere.
If ,as one report indicated, the a/c were level and on RVSM, and if the Go was above the Legacy, many scenari can be drawn. Sudden Decompression is one that come to mind.
Only the FDR/CVR of the 2 will draw some light.