Like in Sword and Shield, any Pokemon transferred in from a different game is only allowed to play competitively with a moveset that it could obtain in the game it's playing in. If Landorus-T doesn't have access to Knock Off or Toxic in SV, (despite it being in Paldea Dex or not, it's still in the game,) then it won't be able to use those moves in competitive.
No matter what moveset it has, if it is transferred from a different game, it will have to go through the "moveset deleter" NPC that will erase your current moveset and change it to the level up moveset your Pokemon would have at its specific level.