Not true, you may file on-line for free at www.safersys.org. What takes time is the initial contact and having a PIN mailed to your home. If you file on-line then you receive your DOT number immediately.
Until the insurance is filed the application will never move forward to be published in the federal register. All applications for a MC number are still published in the federal register and the public has a chance to contest the application. No insurance on file, not initial action initiated for the MC number to be granted.
So why were you advised to wait? The insurance company must file a BMC 91X with the required level of insurance. No BMC 91X no action initiated.
Carriers without interstate commerce when the safety audit is performed will have their DOT number inactivated and be required to start over.