It does - the syscalls are part of the official userspace interface which the Linux kernel promises not to break. They can add new syscalls, options or flags, but can’t break existing ones.
It's still not an explicit guarantee. Actually, Linux the kernel doesn't guarantee or promise anything, it's only distros that try, and those that do promise some compatibility, don't promise all that much. The best promise you can find is like a promise of ABI compatibility within a couple of future releases.
You are extremely wrong, so it's probably worth thinking for a moment about how you became so misinformed and why you feel so strongly that you're not misinformed.
This interface matches much of the POSIX interface and is based
on it and other Unix based interfaces. It will only be added to
over time, and not have things removed from it.
This is literally an explicit promise of the Linux kernel; distros have no influence over the Linux syscall ABI whatsoever.
I think you're perhaps extremely confused about the difference between the userspace syscall -> kernel interface, and kernelspace API/ABI such as out-of-tree kernel modules might use. About the latter, yes, there are no API/ABI guarantees in vanilla Linux.
Expecting something is not a promise, just an attitude they want to have towards it at the moment. I think you are also confusing who is even in a position to promise what. Kernel is not an OS people can use, but only something an OS (distro) itself can use and, given the license, in any way it wants. And so kernel cannot promise or force ABI compatibility or anything really on behalf of any OS that uses it. It's up to the OS, but OSes modify kernels, backport things, build with various ABIs and so on. Look for example at the mess around x32 ABI, some distros had it, some didn't, some had it and dropped it, some had and promised ABI compatibility for some time, but Linus wants to drop it from the kernel (don't know if he actually did it), so they are in a pickle. Read RedHat's application compatibility guide if you want an example on what the best a linux distro can promise wrt ABI compatibility.
