They are both telling the truth. Having gone through 8 Release Candidates, the kernel.org version of 4.2 is considered stable. In about two weeks you will observe kernel 4.3 RC1 there, and the process continues.
Now, consider Ubuntu. Ultimately, the Ubuntu version, will have several changes additions and deletions that are not included in the mainline releases linked to. There are also patches applied to to the mainline for the debian packaging and changelog stuff in addition to kernel configuration settings. There simply must be a delay between the kernel.org version being declared stable and the actual Ubuntu version being created, tested on multiple and varied Ubuntu environments, and initially released. From the Ubuntu perspective the mainline PPA would never be considered "stable".
To some extent the confusion is coming from the terminology used by kernel.org and Debian/Ubuntu, specifically the terms unstable/testing/stable.
The terms used are:
Prepatch or "RC" kernels are mainline kernel pre-releases that are
mostly aimed at other kernel developers and Linux enthusiasts. They
must be compiled from source and usually contain new features that
must be tested before they can be put into a stable release. Prepatch
kernels are maintained and released by Linus Torvalds.
Mainline tree is maintained by Linus Torvalds. It's the tree where all
new features are introduced and where all the exciting new development
happens. New mainline kernels are released every 2-3 months.
Stable After each mainline kernel is released, it is considered "stable." Any bug fixes for a stable kernel are backported from the
mainline tree and applied by a designated stable kernel maintainer.
There are usually only a few bugfix kernel releases until next
mainline kernel becomes available -- unless it is designated a
"longterm maintenance kernel." Stable kernel updates are released on
as-needed basis, usually 2-3 a month.
Longterm There are usually several "longterm maintenance" kernel releases provided for the purposes of backporting bugfixes for older
kernel trees. Only important bugfixes are applied to such kernels and
they don't usually see very frequent releases, especially for older
Many Linux distributions provide their own "longterm maintenance"
kernels that may or may not be based on those maintained by kernel
developers. These kernel releases are not hosted at kernel.org and
kernel developers can provide no support for them.
Debian takes code from upstream (kernel.org in this case) and packages it. See https://www.debian.org/doc/manuals/debian-faq/ch-choosing.en.html
Unstable has the most recent (latest) versions. But the packages in unstable are not well tested and might have bugs.
On the other hand, stable contains old versions of packages. But
this package is well tested and is less likely to have any bugs.
The packages in testing fall between these two extremes.
And then there is the Ubuntu kernel team's ppa, https://wiki.ubuntu.com/Kernel/MainlineBuilds
This last ppa provides the "mainline" , precompiled, for testing purposes.
So, when choosing what kernel to run you can:
- Obtain the source code from kernel.org, apply any patches you wish, and compile it yourself. You can choose:
Mainline - This kernel is the latest and most likely to have bugs.
Prepatch - This kernel is "pre-release" , or in the development cycle one goes Latest (unstable) -> Alpha -> Beta -> Release Candidate/Pre-release -> Stable release (terms vary with project).
Stable - Released code, less likely to have bugs, good for general use.
Long term - Very stable, at the sacrifice of new features.
or 2. Use a "distribution kernel"
Debian / Ubuntu then takes the source code from kernel.org , and applies patches, so the cycle repeats in what kernel.org would term a "distribution kernel".
The Debian/Ubuntu distribution kernel goes through a similar cycle Unstable -> testing -> stable
The kernel used by various versions of Ubuntu is listed here - https://en.wikipedia.org/wiki/List_of_Ubuntu_releases#Table_of_versions
So, at the time of release, Ubuntu is pulling from the "stable" upstream source code (15.10 is using kernel version 4.1).
At the time of this post, kernel 4.2 is the mainstream kernel, and thus not available in the Ubuntu repositories.
You can either compile the mainstream kernel yourself (I prefer this method) or install it from the ppa - http://kernel.ubuntu.com/~kernel-ppa/mainline/
These kernels are provided as is, as a convenient way for you to try the mainline kernel without having to compile it for yourself. It is unsupported and to some extent expected to have bugs.
They list the current mainstream kernel, 4.2, as unstable according to Debian policy (unstable -> testing -> stable)
As the other kernels listed there have undergone appropriate testing and patches they are eventually re-labeled as "stable"