This actually seems like a throwback to jQuery mobile which had a similar way of doing full page transitions. I always thought that was ahead of its time.
I also found a bug (latest Chrome on macOS):
- open https://layerjs.org in a new tab
- navigate to https://layerjs.org/examples.html
- close tab
- reopen tab with cmd+shift+t
- pressing back will change the URL back to https://layerjs.org but the page will not change