This is a great point! You've arrived at the conventional wisdom that I am trying to challenge with the project. Conventional wisdom says that "combat as sport" a la 4E and "combat as war" a la OSR are fundamentally at odds, and I think you've articulated that very well.
I don't necessarily think that is the case. Even in OSR games, combat is not always avoidable. Engaging with the combat system is necessary for progression: treasure and experience are often locked behind nasty encounters, enemies will rise up to present themselves as an existential threat that will need to be dealt with, etc. So I'd say, OSR isn't telling you not to fight, it's telling you to choose your battles wisely and try to give yourself every advantage before the dice start flying.
Trespasser is trying to do the same. Even victorious battles are costly, and victory should be carefully weighed against the benefits of avoiding a fight entirely. it doesn't expect you never to fight, it expects you to have discernment about when you engage, instructing you to choose your battles carefully. It even provides a retreat mechanic to facilitate this.
I think the paragraph you reference is intended for players coming to the game from 5E or other more heroic games, where they are expected to say yes to whatever battle the DM puts in front of them and fight it to the finish. That advice to pick your fights carefully is meant to be a clue that in a player-driven game, not every battle will be meaningful or necessary, and the party will have to be judicious about when it is worth it to fight. The Judge and the game will not make that decision for them.
This is often the first thing people wonder when they look at Trespasser, so I'm glad you've given me an opportunity to talk about it a bit! I often describe the goal of combining tactical combat with OSR principles as a "peanut butter and tuna fish" sandwich. My goal is to make that sandwich delicious, as stupid an idea as it is! 😁 It's been a great design problem at the heart of this experiment that has kept me interested in it a lot longer than I thought I would be! But I think Trespasser has struck a good chord in the current edition.