Try holding down the Option key just after powering on the laptop so you get the Apple boot picker menu. While at the Apple boot picker menu see if the eject key on the keyboard will eject the disc. Another option is to hold down the trackpad button or left mouse button as soon as you power on the laptop. Keep holding the button as the computer boots to see if the disc will be ejected.
Have you tried using another USB port? Is the SuperDrive connected directly to the laptop's USB port without any USB hubs or adapters? A USB-C to USB-A adapter is fine as long as you confirm the USB-A portion works.
If these things do not work, then assuming the SuperDrive has failed, then the only way to retrieve the disc is to disassemble the SuperDrive to access the stuck disc.