I'm referring to this clip from streamer CovertGoBlue. He had pumped his commander (Illuna, Apex of Wishes mutated on top of a 2/2 Zombie token created by Field of the Dead) to 24/24 and then damaged the opponent. While he soon won, why didn't he win the instant after that attack? His commander had dealt 21 damage to the opponent.
The game mode in play is Brawl. Brawl does not have the Commander Damage rule.
The rules that cover the Commander format are in section 903 of the Comprehensive rules. The ones in question here, about format specific win/loss conditions are 903.10:
903.10 The Commander variant includes the following specification for winning and losing the game. All other rules for ending the game also apply. (See rule 104.)
903.10a A player that’s been dealt 21 or more combat damage by the same commander over the course of the game loses the game. (This is a state-based action. See rule 704.)
And the connected State Based Action, in rule 704.6c:
704.6c In a Commander game, a player that’s been dealt 21 or more combat damage by the same commander over the course of the game loses the game. See rule 903, “Commander.”
But these rules apply to the main Commander (A.K.A. EDH) format only, a 100 card singleton format meant to be played as a multiplayer free-for-all. There is a similar subformat known as Brawl, the differences between the two, such as deck size and starting life total, are covered in rule 903.12, and include this rule:
903.12h Brawl games do not use the state-based action described in rule 704.6c, which causes a player to lose the game if they’ve been dealt 21 or more combat damage by a commander.
903.12h specifically removes the commander damage rules from the brawl variant of commander.