systemd-homed, systemd-nspawn as isolation, and the rest of his ideas aren't being held back because they're controversial, it's because OStree/Atomic "lost" more or less the instant Red Hat purchased CoreOS and they didn't need to build their own de-novo k8s compute distro (parts of OStree lived on in CoreOS and are there as a testbed for immutable images for trusted computing in kiosks/appliances/VDI).
systemd-homed, using systemd-nspawn for management, and the rest go directly against the investments which have been made in [fedora-]toolbox, rootless podman, and flatpak. At the time that I left (~2 years ago), "Openshift is the new platform" was the phrase of the day, which means podman "won".
Red Hat/IBM (via the container toolkit, rootless podman, etc) is/was primarily invested in a growth sector, which desktop/server Linux is/was not, and the Container Development Kit, rootless podman, and the rest are direct mechanisms to get containers sold on k8s and openshift.
Lennart's passion projects are great for Linux, and maybe better ideas than Flatpak and Snap. They don't have a business case inside Redhat, though, and there's only so long that even a principal consulting engineer can essentially spend his time on research projects.
I would guess that he wanted to put more effort into an overhaul of CoreOS/OSTree and try to "unify" it with flatpak somehow to get back to "one Linux" instead of split distros for different cases, didn't get traction, and decided to go somewhere he could work on a passion project.
