Off-hand attack tool tip displays attack modifier, even though you don't have the Two-Weapon fighting style class feature.
Further more the Rogue subclass Thief is able to use offhand attack twice in a round, which breaks with the rules of Two-Weapon fighting:
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. You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative.
