Indeed. The very fact they can make whatever they want out of it, and obviously without using any magical fruit powers, means it wouldn't be so difficult to work on and thus cut. Otherwise this story is contradicting itself big time. It's also quite ubiquitous in the world, unlike huge chunks of diamonds. I suppose Oda might have read enough Marvel back in the day and copied adamantium lore (which I still don't understand).

All that being said, it shouldn't really matter to Zoro. He isn't cutting with superior strength. He's following the path, or philosophy, of being able to cut nothing or anything. Once he learns the breath of sea stone, he's able to cut it, plain and simple. Considering how much experience he has under his belt already, it shouldn't take much time even if he hasn't needed to try so far.