I normally punch out when people start quoting the dictionary. But in this case, I'd instead like to quote the 2014 DMG.
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Location-Based Adventures
6. Plan Encounters
After you've created the location and the overall story of the adventure, it's time to plan out the encounters that make up the adventure. In a location-based adventure, most encounters are keyed to specific locations on a map. For each room or wilderness area on the adventure map, your key describes what's in the area: its physical features, as well as any encounter that plays out there. The adventure key turns a simple sketch of numbered areas on graph paper into encounters designed to entertain and intrigue your players.
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Event-Based Adventures
8. After you've created the overall story of the adventure, it's time to plan out the encounters on which the events of that adventure will hang. In an event-based adventure, encounters occur when the villain's agenda intersects with the path of the characters. You can't always anticipate exactly when or where that will happen, but you can create a list of possible encounters that the adventurers might experience. This can take the form of general descriptions of the villain's forces, details of its lieutenants and minions, as well as encounters tied to the key locations of the adventure.
Both of these sections then direct you to read about "Creating Encounters" later in the chapter. This section also provides an actual definition of what an Encounter is in D&D. You'll note that this definition does not appear in the dictionary entry you quoted. I've bolded it below.
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Creating Encounters
Encounters are individual scenes in the larger story of your adventure.
First and foremost, an encounter should be fun for the players. Second, it shouldn't be burden (sic) for you to run. Beyond that, a well-crafted encounter usually has a straightforward objective as well as some connection to the overarching story of your campaign, building on the encounters that precede it while foreshadowing encounters yet to come.
An encounter has one of three possible outcomes: the characters succeed, the characters partly succeed, or the characters fail. The encounter needs to account for all three possibilities, and the outcome needs to have consequences so that the players feel like their successes and failures matter.
The chapter then goes on to discuss Combat Encounters specifically, and here is where we see the idea of the Encounter as a specific unit of play really takes shape. It talks about the Encounter Difficulty, XP Thresholds by Encounter Difficulty, Encounter Multipliers, Encounter Budgets, and Adventuring Day XP Budgets. This is all obviously very specific to the text's definition of an Encounter as a unit of play... as an individual scene in the larger story of your adventure... and how that influences what the DM is meant to do with them, and how to actually craft them per specific guidelines. The chapter then goes on about Random Encounters, which further reinforces many of these ideas, so I don't feel the need to cite any further.
There's a really strong focus on planning these ahead of time, and many references to that in the text. Very, very little time is spent on anything that could be considered advice on how to maintain player agency throughout this... except perhaps if we want to view the bits about "building on encounters that precede it" and similar very liberally.
So yeah... I hope you can see why someone might view the idea of Encounters as being something pretty strongly in the realm of GM as Storyteller based on this, and how, even if you disagree, there's compelling evidence to show the potential conflict between prep and player agency.