Zoro was definitely capable of hurting king before king of hell he just wasn't strong enough to really make use of it. Before koh king was able to overwhelm and overpower him for most the fight
The best tell for zoro being able to hurt king is honestly just looking at kings clothes. With him only being capable of cutting his clothes and therefore king while his flame was off
While his flame was on tho i didn't matter how strong his attacks were but king and his clothes would be perfect fine
- We know zoro cut kings mask spike off panel
- He cut more of it with tiger hunt
- Cut even more of the mask off panel after unlocking koh mode
- Cut his button up with bird dance
- Then finally cutting his wing with dragon damnation
When zoro landed onigiri, tatsumaki, shisonson, etc. You can notice no matter how deadly it looked king and his clothes were fine
Oda basically showing king accumulating damage like a fighting gun character
King was blocking, avoiding, and disarming zoro even before koh