What Toxicroak said is true, but there are ways to limit the amount of damage that is dealt to Pikachu by limiting the amount of damage dealt to the opponent
What you heard was technically true. Modest decreases the Attack Stat, so Pikachu's Volt Tackle becomes weaker - thus dealing less damage to the opponent, and also dealing less damage to Pikachu.. However this is useless, as it lets the opponent get another hit in and even if you kill them, the end amount of recoil you took would be the same as if you had just OHKOed them anyway.
