The 13 comes from the: "enters tapped unless a player has 13 or less life." text in the mechanics of the lands
This means that each deck can only contain one copy of a card on the restricted list, as opposed to the usual four copies of a single card that a deck can have
Playing with Power MTG is a very popular competitive Commander content creator, and they have tried a few no-banlist games to see what performed best
But before he does, he is given a poem that, in part, reads: Elevens hour is over now, The clock is striking twelves
it works with any deck that modifies its creatures
In this way, cardboard begets cardboard, which begets particle board in some strange way