It is certainly possible to achieve straight asymetric flight with level wings by use of rudder. As you say, this would involve the aircraft slipping.

But why would this deflect the ball?

The wings (and therefore the instrument and the central portion of the tube) are level.

The aircraft is not accelerating laterally, so the tube isn't being snatched away sidewayds, leaving the ball deflected by lagging behind.

The balls weight is acting towards the Earth, as per usual, and is opposed by the ball being held up by the bottom edge of the central part of the tube. Again, no unbalanced forces.

So there is no mechanism to cause the ball to be deflected, so it isn't.