Only looking at the most famous battles of any era of warfare will give you an extremely skewed perspective. There were plenty of ambushes, raids, night assaults, and other non-field engagement forms of warfare in the medieval period. The vast majority of medieval combat engagements would have been on a much smaller scale than the famous battles you've heard about, involving maybe a few hundred soldiers on each side. Ambushes were not in any way at odds with medieval concepts of chivalry and honor. Rulers and commanders frequently engaged in such tactics. Naturally, armies on the receiving end of such an engagement would not be pleased about the outcome, but that didn't stop them from attempting to do the same when possible.

I'm not really certain how Hannibal or Scipio specifically were considered in medieval thought, but medieval rulers weren't ignorant of classical military achievements. Vegetius' De Rei Militari was a massively popular and widely read text among military-minded medieval aristocrats. Check out Christopher Allmand's work on the influence of Vegetius in the middle ages.