It appears its activation condition is more intense than expected.

The official Japanese site clarifies the text of “星遺物が刻む傷痕/Scars Caused by the World Legacy”.

(1) All “Jack Knight” monsters on the field gain 300 ATK and DEF.

(2) Once per turn: You can discard 1 “Jack Knight” monster or 1 “World Legacy” card; draw 1 card.

(3) You can banish 8 “Jack Knight” monsters with different names from your GY and/or face-up from your field; send all cards in your opponent’s hand and Extra Deck to the GY.

Source