Yes and no. Yes, because if your starter floats, that means it consists of a lot of air bubbles. The bubbles indicate activity so that starter is probably ready to bake with. BUT, if your starter is on the more liquid side, the bubbles tend to escape quicker and the starter may not float. Even if it produced as many bubbles as the thick starter mentioned above! So that starter is also ready to bake with, but will not float. So with a float test, you can get a false negative, but you cannot get a false positive if that makes sense. And it depends on the hydration level of a starter, as explained.