This is a reanimator-combo deck: they want to discard a legendary creature (Griselbrand or Atraxa, Grand Unifier) and bring it back with Goryo's Vengeance for a haste attack. Thoughtseize proactively strips the reanimation target or Goryo's Vengeance itself before they can assemble the combo. Surgical Extraction can exile Goryo's Vengeance or their reanimation target straight out of the graveyard once you see one, cutting off the combo entirely since it also strips remaining copies from hand and library. Since this matchup is decided by disrupting their plan rather than a damage race, Mutagenic Growth and Violent Urge come out for the extra interaction.