The core collapse itself produces most of the neutrinos. All of these protons are squeezed together with electrons which produces neutrons and neutrinos.
I was referring to the earlier monoenergetic flash. Yes, most should be produced through electron capture in the collapse, but for earlier event to be monoenergetic suggests a specific nuclear process to me.
https://www.sciencedirect.com/science/article/pii/S092765051...
Low energy tachyons would go a little faster, but you've got the additional problem of explaining why neutrinos got emitted in a spectral line.