Okay, I'm sorry, I was completely wrong. (mostly wrong)
It seems that SOME entries acquired the timestamp of the time of import, but not all. In fact, probably not most.
Terribly sorry for the bum-steer, but there are definitely a few entries are incorrect. Those entries don't appear to have any noticeable corruption when viewed in Thunderbird (i.e. no missing timestamps, garbled display, etc).
I also can't be sure if any other recent emails have had incorrect timestamps applied to them during import.