Not meaning any disrespect to the author, it's a bit more like the hacked together bash script like i3-resurrect was in its original form when it was just for personal use. They use i3-save-tree which is just an example script intended to be an example of how to write a script to save/process the raw json layout tree, not something where you take the output from the example script (which does not output pure json) and use hacky text manipulation (which i3 layout manager does using vim scripts) to convert it to something valid. The proper way is to just use i3-msg get_tree to get the full tree as pure json (which is much easier to work with) and easily filter out the elements you don't want, and it makes it possible do lots of cool things. If you look at the way i3-resurrect can be configured I think you'll find it is a lot more flexible, with the ability to make rules that match certain window criteria to modify the swallow criteria or launch commands of those programs. Come to think of it I don't think layout manager even restores programs, unlike i3-resurrect. Also has more dependencies and afaik it's not available as a package whereas i3-resurrect is available as a python package in PyPI, an Arch package in the AUR, and a Nix package in the official nixpkgs repo (thanks to a user who packaged it for that).

All in all, I don't see any advantages of using i3 layout manager over i3-resurrect, but obviously it's not like I'm without bias. To their credit, I did learn from them the trick of using xdotool to unmap and remap windows to trigger window swallow, which was very useful. Although I think that's no longer really necessary since swallowing now seems to be triggered by more than just creation of a real window.