Some of these "use derailleur A with cassette B" setups require the cable to be clamped on the "other side" of the clamp bolt, so there's a different amount of movement per shift, since the cable attachment point is moved. That might be how they go the 11 and 9 to work together?
Standard spacings don't seem compatible:
Who knows if these cog spacings are correct.
9 speed Shimano spacing: 4.34 mm Link
11 speed Campagnolo spacing: 3.85 mm Link
After 8 shifts,
8*4.34 = 34.7 mm
8*3.85 = 30.8 mm
That's one whole cog difference, so after 4 shifts, it would be off by a half shift. It doesn't look good.
It's easy to try if you have a 9 speed wheel, but you'll probably need to use a real 11 speed hub and cassette. Not all 10 speed Campagnolo hubs will work with 11 speed, without hitting the spokes.
For example, from White Industries:
H3 version is Campy 11 speed compatible.
Neither the H2, nor any previous generation White Industries hub will work with Campy 11 speed. The newest 11 speed system requires the largest cog to cantilever over the hub shell and our older hubs were not engineered based upon this design.