I agree with you Cali. So let's see, for the battle to be interrupted, either someone important for Zoro shows up, or someone important for Hodi or someone important overall.
Franky, Luffy, Sanji, Chopper and Robin are either at the Sea Forest or going there, so it's unlikely to be them (All of them important to Zoro). Jinbei and Shirohoshi are the same (Important overall).
Nami, Usopp, Brook, Keimi and Pappug (important to Zoro) are escaping, so it is not likely that they'll interrupt. They could be captured by the invaders, make Zoro loose his focus or something like that (Which doesn't really count as a loss, for it isn't the full skill), but then again, only Keimi and Pappug can actually be captured, and I doubt it, for the same reason why Zoro can't loose: it would make N, U and B's time skip obsolete.
I don't remember what happened to Neptune's army or Neptune himself, but I believe they escaped with NUB, but Neptune might return to save Zoro, which showed marks of great honour. And that could make Zoro loose his focus and protect the king.
The other thing that could happen, which I think it's the most likely, is that the 3 Princes arrive to the palace and interrupt the fight somehow. I believe that's the best hypothesis. Then again, they might interrupt the Hodi vs Zoro fight, but create a Princes vs Zoro vs Hodi, which I think won't last enough for Luffy to intervene (I believe Hodi will fight Luffy, so ultimately, he must intervene).
So maybe a huge explosion could happen in the castle, made by our dear Usopp, providing enough distraction for Zoro to escape with a smirk, leaving Hodi furious. Of course, after Zoro fought on pair with Hodi for a while, serving only to infuriate Hodi more.
