Swift Strike: Genji darts forward, slashing with his katana and passing through foes in his path. If Genji eliminates a target, he can instantly use this ability again. (Damage: 50)
Dragonblade: Genji brandishes his Ōdachi for a brief period of time. Until he sheathes his sword, Genji can deliver killing strikes to any targets within his reach. (Damage: 120 per slash)
As can be seen from above, Swift Strike only deals a fixed amount of damage (50). In addition, if I am not wrong, Genji uses different weapons for Dragonblade and Swift Strike so it can't be possible that you will slash as you dash.
The reason why you are killing enemies instantly is most likely because they are low enough that you can just dash once/slash once and its enough to kill them.
Source: The game itself