I am not persuaded by appeals to authority when easily verifiable math does not support the assertion.
But for what its worth, I find the presumed pressure differentials intriguing. The shape of the projectile should affect localized pressures so it would be fascinating to see the pressure modeling for various cavity shapes (something I would expect to play a much, much greater role in expansion than a 17% difference in the medium's density). I wonder if I could get solidworks to model deformation/expansion...