Yes it really is as simple as that.
If you play an Em shape with a capo on the 5th fret you are playing Am (but with the Em shape). The same for all the other open shapes.
There's a very interesting trick if you are playing as a duo with two acoustic guitars both strumming the same chords. For a bit of variety and to create a much richer and more interesting sound, one guitar plays, lets say, Am in the standard position, and the other plays the Em shape with capo on 5th. (So also plays in Am).
So if your song has chords, say, Am, G, C, player 1 plays those as usual and player two capos the 5th fret and plays the shapes Em, D, G.