... does this also apply to the player's heroes who died in battle, or only to the computer player's heroes?
No, only to your own! It is practically impossible to track the fate of computer players who have lost their heroes.
A simple example:
The computer bought a hero and used it to attack centaurs on the same day! The battle was lost, and the hero disappeared, with exactly the same stats as it had initially!
Sincerely, VDV_forever
