This concerns pressure-treated wood that is likely to shrink quite a bit in width as it dries.
Does the fence shrink in length when the wood dries if you attach the beams to the side of the posts?
Think of it this way. The beams are attached to the posts with angle brackets. When the post shrinks, it pulls the beams with it. And the total length of the fence becomes shorter (number of posts X mm/post). If I have 12 posts and each shrinks 2mm, the fence becomes 24mm shorter. And what happens then with the outermost posts?
Compared to making notches where you attach the beams so the beams meet. Since the beams do not shrink in length, they resist, and the fence will not shrink in length.
