Pour a glass of water and hold it loosely between your thumb and index finger (tightly enough that it won't fall, but loose enough that it can swing freely). Then take it outside and swing it in a big vertical circle. You'll be surprised how slowly you can swing it while still keeping the water in the glass.

Just like the glass of water though, if the plane were to stop rolling, the tea would immediately spill. It has nothing to do with "bending gravity", simply the reactive centrifugal force imparted by the turn itself.

The same reason a pail of water swung around your head doesn't spill. The path cut by the rolling plane in this case is a spiral, as it's a barrel-roll rather than an aileron roll.

So yes, if you stopped at the top of the roll, the liquid would indeed fall out. The reason it doesn't is that the plane is continuing to roll, and the acceleration of the plane away from its upside down position is greater than the acceleration due to gravity. The liquid is "falling" at 9.8 meters per second, but the plane - and hence the glass - is being pushed at a faster rate.

Both the plane and its contents experience the same acceleration due to gravity. Unless the plane is accelerating, the occupants will experience weightlessness (e.g. vomit comet). Of course the plane is accelerating - in straight and level flight this acceleration counteracts the acceleration due to gravity. This acceleration (lift) is provided by the wings, which don't move, so it is always in the same direction (in the plane's frame of reference).

So if you kept flying upside down the tea would eventually fall, right?

No. Eventually the plane would hit the ground, since it is not only falling but also generating "lift" in a downwards direction.

Bob was doing this before the designers of the f-16 were a sparkle in their parents eyes. Bob knew Orville Wright and Charles Lindbergh as friends.

youtube up the video of him flying the f-86 sabre if you want to be stunned.