The reason, is that if you put the rocks in zone 3, this is the "last zone" you will retain shadow-details in.
The sky is 4 stops brighter (zone 7), which is bright, but as you said; It was measured pretty close to the sun, it should naturally lie there, you should still be able to burn in things, need be.
I use the advice in a book call simplified zone system. The basic advice there, is to measure the "darkest shadow-area you wish to retain shadow detail in" and shoot
Later, when moving into more advanced stuff, it advice you to to as above, but check the brightest area you want details in and see how many stops there are between them. Zone 7 is the "brightest zone which still will retain detail").
If your important sky highlights was zone 8 or 9, I would consider n-1 and n-2.