System partitions being non-writable has nothing to do with the vulnerability - it allows modifying the cache of any file that you can open for reading.

Not using setuid anywhere means you'd have to build a slightly more clever exploit, but it's still trivial - just modify some binary you know will run as root "soon".

But... I didn't check, but IIRC the untrusted_app secontext that apps run in is not allowed to open AF_ALG sockets - so you can't directly trigger the vulnerability as a malicious app. Although it might be possible in some roundabout way (requesting some more privileged crypto service to do so).