The pdxinfo file needs additional fields filled out in it for sideloading via a user's play.date account. More info here: https://sdk.play.date/#pdxinfo. The docs don't indicate which fields are required, but I believe it might just be bundleID. I would recommend included that as well as name, author, and version at a minimum.
Is that something we can edit ourselves and get the game running on the console? Or does it need to be done by the dev?
You should be able to open the .pdx file (it's really just a zip file I believe), edit the pdxinfo file to add the necessary fields, and package it back up into a .pdx. Then adding the game via the Sideload page in your account would work.
FWIW, I believe you can directly add the game via USB without a valid pdxinfo file if you'd rather go that way.