The combat rules are not the game.
They're part of the game. And prescriptive.
So are the rules for building PCs (determine
these six stats; choose race; choose class; calculate various bonuses; etc).
So are the rules for resolving ability/skill checks and saving throws.
The game as a whole just describes ways to set up the world and run things, it doesn't force a way on the DM and players. They include optional rules and often let it be known that the rules are just guidelines.
Optional rules aren't thereby non-prescriptive. If you opt to use the optional flanking rules, then those rules
prescribe a certain circumstance in which advantage is gained - and
advantage is itself a prescriptive rule (roll two dice, keep the best). Neither the flanking rule nor the advantage rule
describes anything.
I could say that the rules of chess "just describes ways to set up the pieces and move then" - but that wouldn't show the rules of chess are not prescriptive.
I think there's an argument that the notion of
descriptive rule set is incoherent - that the whole idea of a rule is to prescribe a norm or mode of conduct. But without going that far, the rules of RPGs are clearly prescriptive in all sorts of ways.
Gygax's PHB, p 7:
As a role player, you become Falstaff the fighter. . . . You act out the game as this character . . . You interact with your fellow role players, not as Jim and Bob and Mary who work at the office together, but as Falstaff the fighter, Angore the cleric, and Filmar, the mistress of magic! . . . [O]ne player must serve as the Dungeon Master, the shpaer of the fantasy milieu, the "world" in which all actions will take place. The other participants become adventuerers by creating characters to explore the fantastic world and face all of its challenges . . . By successfully meeting the challenges posed, they gain experience and move upwards in power . . .
From the 5e Basic PDF, p 2:
In the Dungeons & Dragons game, each player creates an adventurer (also called a character) and teams up with other adventurers (played by friends). . . . One player, however, takes on the role of the Dungeon Master (DM), the game’s lead storyteller and referee. The DM creates adventures for the characters, who navigate its hazards and decide which paths to explore. . . . Each monster defeated, each adventure completed, and each treasure recovered not only adds to the continuing story, but also earns the adventurers new capabilities. This increase in power is reflected by an adventurer’s level.
These are instructions on how to play the game: "You become . . .", "You act out . . .", "You interact . . . not as . . . but as. . .", "one player must serve as . . .", "By successfully meeting posed, they gain . . .", "each player creates . . . and teams up with . . .", "One player . . . takes on the role of . . .", "The DM creates . . .", "This increase in power is reflected . . ." - these are all prescriptions.