Doesn't seem to be the error in what he did actually.
The margin property can be used in a number of ways:
1. set 1 value which will mean you set a specific value for all sides.
2. set 4 values. values go: top right bottom left (clockwise)
3. set 2 values. values go: top and the rest.
It's the lazy way of having to type margin-top, margin-right, margin-left and margin-bottom and i'm glad they made that.
In any case, you have to apply both margin-top and margin-bottom to get the margins between paragraphs you want or you can just use line-height but I found that worked less efficiently on some books so stick to margins.