Well, Timaeus didn't actually go to Corsica; (since we don't have his history, and most of what we know about the specifics is Polybius being indignant about How Wrong Timaeus Is, so it's kind of hard to tell what was going on.) But he probably compiled his information from other geographers and then included it in a digression about the island. Which is what Polybius is so proud of: he didn't depend on books; he went there and caught the animal and ate it and saw (and tasted) for himself!