It's true that by declaring a function member as static, you make it independent of any particular object of the class and the function can be called even if no objects of the class exist, so you can get a pointer to that function.
but... a static member function have a class scope and they do not have access to the this pointer of the class so they can only access static data member (and other static member functions and any other functions from outside the class).
➜ it won't work as you won't be able to perform this for example (_leds being a private instance variable, non static)
for (int i = 0; i < 8; i++)
{
digitalWrite(_leds[i],number & (0x01<<i));
}