It is possible to damp the amplitude of a harmonic oscillation without changing its frequency. That was my proposition.
Again, I suspect gas suppression has a large part in achieving accuracy with a can. As you implied, it's an internal pressure thing. If a can reduces the exit pressure of gas around the heel of the bullet from 12k psi to 2k psi, that would contribute less stability disruption to the bullet that running can-less.
