I do not understand why the game is marketed as a Strategy game, even the original 1990 game is marketed as a Strategy game, it defies any kind of logic or sense.
In King's bounty you exist in a static world, no-one is competing with you. Like any other RPG monsters sit around in their designated places waiting for you to come and kill them. You do not gain any territory by your conquests, you are simply 'clearing areas' as you perform quests for the legitimate landowners, like any standard RPG.
With Chess, the board is the entire game, each piece represents a controllable character who's objective is to control a section of land and your opponent is as equally active as you. In KB you just have your main protagonist, nothing will happen at your eastern border while you are exploring your western border.
The individual battles in KB are like mini-chess games (but interesting), so I guess you could argue that the game has some minor strategy elements, but this is not the whole game, this is merely how combat is resolved, and pieces on the KB chess board don't compete to dominate territory, they fight like monsters do in any RPG (chess does not have archers and magic, etc, each piece is solely designed to dominate territory).
The game as a whole is primarily a traditional RPG, the only difference being that when your avatar gets attacked by monsters you are zoomed to a chess board where your accumulated army fights another accumulated army - which is not greatly dissimilar to individual units attacking each other in a normal RPG, one just has 'number of units' instead of a 'HP bar'.
Because the battles are about using units to solely deal damage rather than solely dominate land space the battles in KB are therefore more tactical than strategical, as they are over in a few rounds, there is no possibility of stalemate, and decisions are often reactionary rather than pre-planned. By becoming extremely experienced with the game, then you could pre-plan for every encounter possible, but as a first time player (the majority of gamers) the options for strategy are minuscule and I think if someone bought the game hoping for strategy then they'd likely be hugely disappointed but if someone bought it as an RPG then they'd not even notice any strategical elements.
I have no idea why people are marketing the game as a strategy game and everything you say can be applied to practically any game, even things like Tetras.