Why would he forget things that happened before he died, about a week ago?
I think Jirix is deliberately lying when he says it was more than one elven wizard, so as to not let anyone know that just one wizard did all that damage. And he is leaving out any mention of the phylactery because it is secret.
What I find interesting is that we, the readers, are seeing the same scene with the phylactery missing. Even though we know for a fact that it was there.
It's as if the author is telling us that what we see in crayon pictures is not always the truth, by showing us an exact panel we've seen before but with one crucial change.
Not what he recalls. What he says he recalls. Huge difference.