On your final note, it's not really the same kernel version throughout, and at times they have bumped up major upstream kernel versions mid-LTS, but even disregarding that, they are constantly applying security patches (think of it as minor version increments with bugfixes from the future).
So sure, you're running kernel 5.15.0 on Ubuntu LTS 22.04, but maybe you started at 5.15.0-36 and after a few months of incremental upgrades you'll be running 5.15.0-85.
On your final note, it's not really the same kernel version throughout, and at times they have bumped up major upstream kernel versions mid-LTS, but even disregarding that, they are constantly applying security patches (think of it as minor version increments with bugfixes from the future).
So sure, you're running kernel 5.15.0 on Ubuntu LTS 22.04, but maybe you started at 5.15.0-36 and after a few months of incremental upgrades you'll be running 5.15.0-85.