You are not completely mistaken, but: This would be true for higher Level Pokémon, but since the target is only Level 1, any base Defense below 50 will result in the same effective Defense: 4. Thus, the Ledyba could just as well have been a Delibird, Combee, Yanma or Ninjask, since these also have base Defense < 50 and are 4x weak to Rock.