Does the bonus action attack as part of the Soulknife’s Psychic Blade feature (the one that does 1d4 damage) add your ability modifier to the damage? Attacks add your ability modifier to damage as appropriate to the type of attack (melee, ranged, finesse) unless it specifically tells you it doesn't.

— Dan Dillon is the Least Turnip 👥 (@Dan_Dillon_1) April 15, 2020

So then the bonus action attack is not considered “two weapon fighting” as described in chapter 9 of the PHB? Correct.

— Dan Dillon is the Least Turnip 👥 (@Dan_Dillon_1) April 15, 2020