Well luckily the Hekate GitHub is very informative and answers your question for you. Quoted directly from the page itself (which I highly recommend you do if you have any questions about Hekate in general for the future):
That's exactly the description I was confused about, but it seems to replace "reboot_payload.bin" so I think that answers my question, without that setting rebooting to payload doesn't get me back to Hekate from Atmos.