(1) The startup time for IMO is 13us, this means that the IMO requires 13us to stabilize after the device is started.
(2)In case you want to transition from low power mode to Active mode (IMO). The IMO will require stabilization time of 13 us to stabilize, since IMO is not active in low power modes.
When the transition from 24MHz (USB, Active mode) to 24MHz (non USB, active) occurs it would not require a stabilization time of 13us if the clock source remains the same. The transition will be immediate.