Can’t argue with that, however if the fiber reads detonation of the high explosive, there will be negligable x-ray production at that time.
The time scale at which things occur in a nuke is amazing. Triggering of various data collection systems is calculated very precisely, and it doesn’t always work out perfectly.
I worked in the Nuclear test field in the early 90’s when we were conducting the final tests. There was one fiberoptic instrument that I was aware of (having worked on it a bit), but it was never fielded prior to the end of testing.