He makes the contention that the basic activity of RPG is playing a role. Where he stumble is giving this example and claiming everything is extra.
Everything else is NOT extra. In fact if wasn't present people would have quickly get bored of D&D. As a hobby we are long past where the rule system alone is enough to sustain years of gaming. In fact it wasn't really true "Back in the day" either.
Fighting Man Level 1
ST 12, IN 8, WI 10, CN 9, DX 10, CH 9
That's your character and your role, right there.
I am NOT advocating that creating a background, a personality, speaking in funny voices while creating a character is necessary. But in the best campaign there will come a point that your character does have a background, does have a personality (even it is a shadow of yours), and yes may even have acquired a funny voice.
This is a side effect of playing RPGs and arises out of the fact that character has continuity between sessions. Unlike a game of Dark Tower, Talisman and other boardgames a RPG character grows and develop.
You do have a choice of either front loading the process by making it a part of character creation or back loading it through play. There is also a problem where some advocates of front loaded characters sneer at those who just want to just roll and play.
Make no mistake, if you look at the classic long term campaigns from Blackmoor, Greyhawk to today. You will find that nearly all of them have a deep background arising out of what the players did. That the referee choose to build on the consequences of the player's actions and followed through to keep their interest engaged. Getting their interest and keeping it engaged is one of the primary keys to a fun long-term game.
I am keenly aware of the issue because I run sandbox games. While I feel this is important for just about any style of RPG play, it is critical to a successful sandbox game. The players come through the orc village in hex 1312 and kill everyone. Two game months later again they come through again.
As a referee you have to decide what were the consequences. Even if you decide there is none, your campaign now has a history and a background. The Orc Village in Hex 1312 is no more.
What makes OD&D good isn't the rules alone or the roles created by the classes. What make OD&D good is that that it gets out of the way when you need it out of the way. That you don't have to look up Rule 13.2.2 while trying to convince the Duke to give you more supplies for another go at the Majestic Fastness.