[Vuejs]-I thought methods are not reactive but why in this example the method is reactive?

In the context of Vue, values’ being reactive means that their state (properties) are tracked and their setters are intercepted to trigger re-rendering.

Related part in docs

In contrast, methods of components are not reactive, because changing their properties doesn’t trigger re-rendering.

In your sample, you don’t change properties of iAmAMethod method, but you change the counter data which is reactive. That’s why Vue re-renders the template.