Ok .. don't worry about that second paper. It seems to go on about some other theory about how comets are formed. We can do without that at the moment.
It seems that there are several theories about the Tempel 1 Impactor generating more energy than was expected. Some papers talk of chemical exothermic reactions caused by the comet chemicals upon impact, others talk of rebound effects, etc.
Interesting also, (I note), is that twenty and ten seconds before the impact (that is, ~ 200 and 100 km distant from the nucleus), the Deep Impact impactor suffered two collisions with large dust particles that disturbed its orientation (A’Hearn et al. 2005b).
So things were happening before the impactor impacted !!
This could also explain the pre-impact flashes.
It doesn't have to be electrical discharges, Alex.
Cheers