Thanks for sharing that, thats wonderful.
It only works for me if I set my desktop to light mode. When set to Dark Mode the selection box goes like below and it will not dynamically change the background.
View attachment 856522
However that selection box is perfectly fine in Light or Auto mode (although then the thumbnail doesn't match the one used in dark mode) and will change the background.
View attachment 856523
I compiled the "heic generator" from Mojave 10.14.3 with Xcode 10.3 (and Swift 5), however it's weird since I set both the thumbnails as the developer explained in his "json" examples, probably the developer focused more on Mojave than Catalina. For example I tested it on Mojave and it has the Mojave desert animation, but if I pick separately Dark/Light/Dynamic they work with Catalina.heic .
Moreover I also specified the time clock but I noticed that sometimes it is not so much accurate, probably there are some workin progress on source code or missing functions not handled, but as a beta dynamic wallpaper for a beta macOS let's say it works in an acceptable way.