While it's true Luffy always does what he says he will, that barely ever stopped outside interference in his battles. Luffy had an argument with Law about him wanting to beat Doffy, Law still made a serious contribution to Doffy's defeat and that didn't make Luffy's role in defeating Doffy insignificant and that didn't impact Luffy's words either. So I don't see why it's suddenly a problem when it's Zoro that's getting involved.
And Luffy's words aren't absolute either. He said he'll beat Big Mom in this war and she went down without him ever landing a hit on her. He also said he's gonna cut down Kaido a while back and I doubt he'll suddenly transform into a swordsman. Half a dozen people already got involved in Kaido's fight, this fight is never meant to be a complete one-on-one for Luffy. He was always meant to be the primary contributor in Kaido's defeat, not the sole contributor.
All of this doesn't mean ZKK happens though—ZKK ain't happening but Luffy's role in defeating Kaido is hardly an obstacle for ZKK theory.