How do I know if an embedded systems project is using an embedded operating system?

How do I know if an embedded systems project is using an embedded
operating system?

I followed these instructions and was successfully transmitting IEEE
802.15.4 frames on a GINA Mote. I know it was working because I have a
packet sniffer that captured transmitted packets.
Here is the source code:
https://github.com/openwsn-berkeley/openwsn-fw/tree/d1ec9982fbc101061b4bc70bde239e54cd1367c4/firmware/openos/bsp/boards/gina
I'm a little confused how and why it's working though. Is this code
loading an operating system (like RTOS) on the Gina mote or is this
project OS-less ?
I'm looking for a solution that does not require an OS / bootloader.
I would appreciate if one of the experts in the community could weigh in
on this.