WHY is Soulknife’s second attack NOT Two-Weapon Fighting?

Ask about Soulknife rogue, D&D 5E

Hello, I know there is a question "Do you add your ability modifier to damage for the Soulknife’s second attack?" with 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. I am not english native so maybe there is something crucial and obvious escaping me.

Also, 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