Not only does the number of swords Zoro uses matter, but the quality of the swords matter as well. During the Killer fight he said he would have to "make do" with using Killers scythe when using Rengoku Onigirl. An attack that mind you, he utilized Haki with, while Killer was not using haki of his own to defend himself. The most the attack managed to do was KO killer. Now imagine if that same scenario played out, but Zoro had Shusui at the time instead of a regular scythe. Killer would more than likely still be recovering from that injury.
On the rooftop, Killer said that the fight would have been different had he had his signature punisher blades.
These two instances tell us that the quality of the blade matters a lot.
Take away Zoro's swords completely, and he's not even a fraction as strong as his is with them. People can argue Tatsumaki all they want, but without the lethality of the Swords in play, the attack is nowhere near comparable to what it can achieve with swords.