That indicates whether liblxc supports cgroup2, it doesn’t tell you what your host uses. For that, you’ll want to look at /sys/fs/cgroup and what it contains.
A pure cgroup1 setup would have per-controller directories and no unified directory.
A hybrid setup (most common) will have cgroup1 + unified directory.
A pure cgroup2 setup will only have a unified tree (similar to content of unified directory).
My understanding is that Debian 11 uses pure cgroup2, though maybe not for folks upgrading. In any case, something to keep an eye on.
Setting systemd.unified_cgroup_hierarchy=false on the kernel command line should force systemd back into a hybridge cgroup1 mode.