They are running their LTS distro's LTS kernel, but that us not an upstream thing
On an LTS, you'll be running a Canonical kernel, or a Red Hat kernel, or a SuSE kernel, or an Oracle kernel, or...
Each will have different backports, different hardware enablement, different random patches of choice, and so different bugs and problems.
Unless were evaluating the security of a particular distro release, mainline is what is Linux and will ultimately be the shared base for future releases.
On an LTS, you'll be running a Canonical kernel, or a Red Hat kernel, or a SuSE kernel, or an Oracle kernel, or...
Each will have different backports, different hardware enablement, different random patches of choice, and so different bugs and problems.
Unless were evaluating the security of a particular distro release, mainline is what is Linux and will ultimately be the shared base for future releases.