I.. am not sure why it says 0 bytes. The file is actually 11.4 MB and that's what gets downloaded.
You can sideload the entire grelb.pdx folder as a single item. It can be zipped separately but there's not really any need to.
I also opened a feature request in the forums asking for some official guidelines, and maybe proper support for pdx.zip (both for tooling and sideloading to the device itself) to ease the process for users. https://devforum.play.date/t/standardize-and-document-way-to-distribute-games-fi...