I'm impressed he flicked a knife and displayed that much power and control with little to no effort. It is more about getting a grasp of his most basic abilities. The type of stuff he can do (very) casually while handicapped. The smaller scale feats are just as noteworthy as the larger scale ones.
He was handicapped?
That's my point: was it that much power even when he only used a knife? Given his current level it's obvious a tiny building requires zero effort (Luffy smashed a several-floor building all the way back in Arlong arc).
As for control, it's not like Zolo did something particularly skillful, either. It's a straightforward air slash and the samurai guy sits not close to the others so hitting only him is quite easy (the air slash does not have much AoE anw). Had it been shown Zolo cut an ant in a clustered nest faraway or deflected some bullets with complex trajectory then yeah, good feat it is. Maybe next chapter we will know but for now...
I found this feat pretty...obvious from him. Like if you tell me Son Goku can break a wall I'd say ''well, duh''. How badass it looks is another matter though.