Why is Soulknife's second attack not Two-Weapon Fighting?

Question

I know the question Do you add your ability modifier to damage for the Soulknife's second attack? with a reply being "Yes you do because it is not two weapons fighting". This is not if, but why.

Recently I encountered this problem while playing Soulknife and pretty much my entire group was like "it's in off-hand, it's two weapons fighting because you are literlly using two blades, it should not apply".

Why wouldn't Soulknife's second blade be considered Two-Weapon Fighting while using bonus action to use second blade?

Is it because both are not held at the same time or something like it? Some wordplay on second and two not being the same word? I honestly do not understand why two weapons are not two weapons fighting.

Am I correct to assume that if second blade is not considered two weapons fighting, then it goes like this:

First blade: Attack roll 1d20 + prof + mod; Damage roll 1d6 + mod

Second blade: Attack roll 1d20 + prof + mod; Damage roll 1d4 + mod

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I am not English native so maybe there is something crucial and obvious escaping me.

Answer 1

Two-Weapon Fighting is a specific thing

The Soulknife feature states:

[...] After you attack with the blade, you can make a melee or ranged weapon attack with a second psychic blade as a bonus action on the same turn [...]

Meanwhile, Two-Weapon Fighting states:

[...] When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. [...]

TWF is a specific maneuver, and a specific thing you can do only when certain conditions are met (taking the Attack action and making an attack with a light melee weapon wielded in one hand and later having a different light melee weapon in your other hand). The Soulknife feature is not the same feature. If it were, it would have explicitly stated as much. It is actually more similar to the Monk's Martial Arts feature:

[...] When you use the Attack action with an unarmed strike or a monk weapon on your turn, you can make one unarmed strike as a bonus action. [...]

Neither Martial Arts nor Soulknife are Two-Weapon Fighting because TWF is a completely separate feature/ability with its own rules. If something were to use Two-Weapon Fighting, or modify it in some way, this would be explicitly stated. The Two-Weapon Fighting Style and the Dual Wielder fest do precisely this:

When you engage in two-weapon fighting, you can add your ability modifier to the damage of the second attack.

[...] You can use two-weapon fighting even when the one handed melee weapons you are wielding aren’t light. [...]

These both explicitly mention Two-Weapon Fighting and how they modify it. Soulknife never even mentions the feature because it is entirely separate. Another way to phrase that it is a different rule entirely (credit to Ryan C. Thompson) is that a Soulknife could still use their bonus action attack even if the rules on Two-Weapon Fighting didn't exist.

Answer 2

Two Weapon Fighting is available to everyone.

TWF is a rule that is given to every character, no matter their class, so when you make a bonus action attack via the TWF rule, without a particular class feature, you don't add you ability modifier to the damage of the bonus action attack.

Psychic Blades is only available to the Soulknife Rogue

Psychic Blades is a unique ability with its own rules. It is an entirely different bonus action than the one given everybody via TWF. The rules for Bonus Actions say:

You can take a bonus action only when a special ability, spell, or other feature of the game states that you can do something as a bonus action. You otherwise don't have a bonus action to take.

Everyone is given TWF, only Soulknife rogues are given Psychic Blades.

Answer 3

Psychic Blades is a specific feature for Soulknife rogues that grants several benefits. This includes a new bonus action which is similar to, but distinct from the one provided by two-weapon fighting, which is available to all characters. The rules for one don’t apply to the other.

Psychic Blades is different to two-weapon fighting in many ways:

• It modifies the Attack action, rather than just using that action as a prerequisite (i.e. manifesting a blade is part of that action);
• The character has to have a free hand for each blade they wish to manifest;
• The psychic blades have fixed damage dice (1d6 and 1d4, respectively) and deal psychic damage;
• Both the primary and secondary attack can be melee or ranged attacks (there is no restriction listed on the first one, and the bonus attack specifically says you may make a melee or ranged attack);
• The second attack is not modified such that it does not add the rogue's ability modifier to its damage.

The two attacks are separate, and your interpretation of how their attack and damage rolls would be made are correct.

Having access to Psychic Blades doesn't mean you can't use two-weapon fighting if you want to. You can still carry two regular, light melee weapons and use two-weapon fighting as normal - including not adding your ability modifier to damage on the second, bonus action attack.

My reading of both rules even suggests you could combine the two, in a limited way, since the primary Psychic Blades attack is part of the Attack action, and the blade itself is a light melee weapon. A Soulknife rogue with a shortsword could manifest the primary psychic blade in a free hand, make a melee attack with it, and then use their bonus action to make a regular two-weapon fighting attack with the shortsword in their other hand. If so, the rogue would not add their ability modifier to the damage of the second attack, because they would be using the two-weapon fighting rule, not the bonus action granted by Psychic Blades.