Donc il est faux de dire que les noeud mirroir (full node) ne gardent que les 256 derniers blocs par défaut.
Ils gardent tous les blocs entier depuis le genesis, mais élaguent leurs états.
Je pense que mon incompréhension venait de là.
La doc de substrate est trop ambiguë à ce sujet:
Although older blocks are discarded, full nodes retain all of the block headers from the genesis block to the most recent block to validate that the state is correct. Because the full node has access to all of the block headers, it can be used to rebuild the state of the entire blockchain by executing all of the blocks from the genesis block