Zoro used a Santoryu technique; unless shown otherwise, he hurt him with all three swords. That is the default assumption given that all three swords were used.
Instances when Zoro used Santoryu to produce one slash:
- Cutting the Pacifista
- 1080 Pound Cannon vs Pica's Golem
- Ichidai Sanzen Daisen Sekai vs Pica's Golem
- Hardened Slashes vs Pica's Golem
Again, until shown otherwise, all 3 swords produced the slash that cut Kaido.
Because Hiryuu Kaen is a stronger attack than Tatsumaki?
Hiryuu Kaen is Zoro's strongest Ittoryu technique, and Tatsumaki is a technique that he mostly uses to clear fodder.
While Zoro was more drained by Hiryuu Kaen than Tatsumaki (he probably poured more haki into it). Even before the timeskip, Hiryuu Kaen was a stronger attack than Tatsumaki.