[tsimionescu]

Looking into this a little, AdS/CFT correspondence is only a conjecture, not a proven fact. More importantly, there are no AdS/CFT solutions that actually work with 4-dimensional curved space time, the working models are all in known non-physical geometries.

[zachf]

It’s a very strongly supported conjecture, but it has not been proven rigorously. As I said it would be extremely surprising if It were not true because of a frankly absurd amount of evidence. The second claim you make is false though— 3D holographic CFTs would be dual to a 4D quantum gravity theory.

Additionally, this conjecture is actually experimentally verifiable—you can set up strongly coupled systems in the lab and study the string theory description, and compare the predictions.