GPL violation?

The vast majority of Android code is AOSP (effectively Google) written/copyrighted/owned, so they can do whatever, regardless of any licenses they (as the copyright holder / author) may or may not choose to grant you as an end user.

Furthermore, the vast majority of Android isn't GPL licensed at all.
It's Apache 2 licensed, which has no source release requirements.

This is why other vendors don't release their modifications to Android, in spite of not using bare AOSP.

This non-requirement to release the source was a day one requirement of Android - it's a large part (along with it being free) of why Android is a commercial success. Vendors were scared of GPL.

For the small quantity of stuff which is GPL, you can request the GPL sources and get a tarball.
I've done it, there's just basically nothing interesting in there, besides the Linux kernel itself.
(in addition to the kernel, there's some external cli utilities for talking to the Linux kernel, they - cumulatively get a few dozen patches per release, often just updates to (better) work with the Android build system, and thus useless outside of an Android build)

The Android forks (GKI/ACK) of the Linux kernel are still developed in the open in AOSP.
So you can simply follow those along live on AOSP gerrit instance.

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Wrt. the article itself. This is basically a nothingburger.

Not great, but also doesn't really matter - it affects mostly GrapheneOS/Calyx/LineageOS custom roms - they'll be releasing some updates 3 months later than previously...

Android for the majority of its lifetime has had *yearly* source releases to AOSP.
Android 9/P, 10/Q, 11/R, 13/T - were released, and then a year passed until the next version was released.
Occasionally there would be an extra point release in the middle of the release cycle (example 8/O 8.1/O MR1 and 12/S 12.1/Sv2 and 14/U).

btw. Honeycomb source release was delayed by a year.

The past <2 years (since Android 14 QPR2) have switched to a quarterly release cycle (though A16 QPR1 was delayed enough that it basically came out just before A16 QPR2 did - make it quarterly only in theory, and basically nearly half-yearly in practice).

Now they're switching to twice a year...

You'll note this is still better than the overall average frequency of Android source releases (since inception) which is I think at the moment averaging around 3 quarters.