The zip isn't formatted correctly to sideload. It needs a "top level .pdx file" to do that.
The intention was that you would upload the pdx file itself, not the zip file. But I didn't realise you could also upload zip files, so I didn't make that terribly clear.
Tbh, I should also have separated the source into its own download anyway. I only didn't because I had something like 15 minutes to upload before I ran out of time. But I've corrected both of these issues now. Thanks for letting me know!