I'm A Banana
Potassium-Rich
If, rather than "determine X", what is meant is consider a range of possible Xs (as per @I'm A Banana's post not far upthread), then it's not hard to write that instead of what has actually been written.
Agreed!
Also, I think the instructions are really two different play modes, so they're really talking about two different things.
First, you have the "preparing the adventure" mode. That's a play mode that the DM usually does on their own. Imagining the arc of the coming session, creating some narrative or spatial branches, giving future you enough info to be a good improviser - that's considering a range of possible encounters. You'll use things like random tables, lists of monsters you'd like, your PC's and their capabilities, notes from previous sessions, etc. Some DMs need a lot of this, some DMs need a little, and how much a DM enjoys this phase is up to personal preference, really. For this mode of play, it'd be good if the DMG included like, a detailed way of doing it, and an expedited way of doing it. Here is also where the game could talk about things like random encounter charts, or how to "theme" a dungeon, how to tie character motivations to obstacles. Act structure might also be a useful touchpoint (and I'd love if we talked about things beyond 3-act structure! Like Pledge/Turn/Prestige, or like the 4-panel structure of Kishtōenketsu or how emergent storytelling works). Figuring out your "villain" could be a nice thing here.
Then, you have the "running the adventure" mode. In this mode, you're letting the players lead the charge and using your preparation to help guide your response to them. Here's where you might talk about the "theater" of TTRPGs, about tone and voice and "say yes." When to hand out Heroic Inspiration, how to make NPC's on the fly, how to deal with "ruined plans," etc.
These things are very distinct exercises, and someone can be VERY GOOD at one of these and VERY BAD at another. Someone who is good at running but not prepping, for instance, might be a very good improvisor with a knack for knowing when to call for checks and how to play with the PC's intentions. Someone who is great at prepping but not running might build grand plans that never see the light of day in an actual session. Probably the latter skill is more important for a DM, but the former can help make the latter easier.