Yes, you are correct, you just zip up the contents of the release folder (for next). For Spectrum +3, you just need the .dsk file.
Both vesions should ideally come with a readme.txt file.
The "release" checklist (which you'll see when you try to submit a game has the following optional suggestions (items 9 + 10):
9 -If submitting 8-bit versions, then this acknowledgment is appreciated. "Thanks to Andrés Samudio, Uto, Tim Gilberts & Stefan Vogt for helping bring DAAD (used in 8-bit versions) to the community." 10 - An optional plug of Adventuron is appreciated. "Learn to write your own text adventure game (in an hour) with Adventuron, https://adventuron.io/documentation/tutorial-a.html "
As far as the format of the README.txt goes, you can use the template of the readme in the zipfile download here (scroll to bottom to find download):
https://adventuron.itch.io/excalibur
Uto is the unsung hero of this jam because without his DAAD Ready package and Maluva (the plugin to display graphics) then the 8-bit mode would be a bit dull. He does this for his love of the genre, and I think he absolutely deserves to be acknowledged.
Regards,
Chris