Why does the isError function not determine whether the passed type implements the interface, but instead uses whether the type is error?

`// isError returns whether or not the provided type implements the error interface
func isError(typ types.Type) bool {
    if typ == nil {
        return false
    }
    // 获取 error 接口的类型
    errorInterface := types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
    // 判断 typ 是否实现了 error 接口
    return types.Implements(typ, errorInterface)
}`