It basically is, but this kernel is much modified by modem maker, SoC maker and device and component manufacturers.
They almost always do dirty low quality patches just to make one device work with Android and not care about sending them to upstream (mainline) kernel or even about compatibility with anything but their Android version.
https://not.mainline.space/ - example of OnePlus 6 having more than 5,600,000 lines of code difference from normal Linux kernel. And is still considered pretty close compared to most phones.
They are publishing their version of the kernel. The problem is that this kernel is so much modified and dirtly patched it is useless to run anything other than Android.
And many device drivers for Android are now proprietary blobs in Android userspace outside of the kernel code.
I think that in the Android kernel (for your device) there are binary blobs for hardware drivers. Which are not in mainline and most of the times not even available anywhere.