Yamato deals 100% extra damage to big monsters and summoned units.

Big monsters are things that take up the entire field, such as group snake or world boss.

Summons are anything with a white health bar, such as clones. Even things revived by orochimaru KT mystery count as summons.

The text description we have is a mistranslation. The passive do not buff anyone else. He does not deal extra damage to jinchuriki.