I finally got around to investigating the issue with downloading this project as a ZIP archive instead of cloning the repository. Apparently, if a repo is using Git LFS to manage large files, the Download ZIP button downloads the repo with the pointer files and not the actual large files that those pointers are referencing. This is why downloading the zip results in a "Failed to load map" error, and why none of the assets show up in the content browser.
Starting today, every release will come with a working ZIP to download here from the Releases page, and I'll direct users here from the README.
Thanks to @JaegeunJung, @klixoft, and @SirVoxelot for the reports.