It’s not entirely correct.
Chainmail has different attack matrices for missile and melee in both of its combat systems (“Troop Type” and “Man-to-Man”).
When Gygax decided to focus on “Alternative Combat System” (d20-based) in Supplement I: Greyhawk, it also inherited separate melee and missile adjustment matrices against different armor types.
This system was also used as a basis for the AD&D ruleset, but it was now spread across two books. While the Dungeon Master’s Guide may give you the impression that the attack matrix is the same for all weapons, you should also take into account the “Weapon types, general data, and “to hit” adjustments” tables in the Players Handbook (p. 38), which give to-hit modifiers for each melee and missile weapon type.