With the latter, I agree.
And no, composition is not all about The Rule of Thirds. It's about what works in a painting. It's a much more practical matter.
The Rule of Thirds is generally understood to mean that, if you divide your painting into equal thirds, both vertically and horizontally, the image that is your center of focus should fall along one of the intersections of the grid you've established.
In the painting above, though, notice that the tree that is the center of interest doesn't quite line up right. It's off to the right of the right vertical line, and the horizon pretty much bisects the middle of the painting, horizontally. Hmmm. Does that mean it doesn't work as a painting? Look at the original painting below to compare.
If this painting were cropped to align nicely along one-third lines, one way of doing it might look something like this, below.
Now, let's say we wanted to line up the upper right lines of the grid through the center of the tree, in its upper part, and recropped the painting's image accordingly. The tree is, after all, the center of interest of the painting.
Let me amend that — you will have more interesting paintings. And here's to more of those!