Those mountains are rather hills. Compare the trees on top of them and the houses beneath. Those are not Mountains.
The castle, which has 5 floors, is already comparable in height to those hills. And note how this castle is already standing out and seen even from afar from Onigashima's perspective.
The scan you posted is also interesting. Take a closer note:
the tree on top of this "mountain" makes up half of it's size. Compared to Zunisha, this random tree might as well be around 5km in height.
Though don't get me wrong, I can see Oda randomly assigning a size to the flower capital of like 10-20km. I'm just saying that Oda is rather terrible when it comes to depicting these sizes, as he seemingly isn't aware of how big 10-20km actually is visually, lol.