Hacking is improvising, no one size fits all answer. Each exploit leads to making different choices based on the vulnerability, its surrounding code, available resources & architecture specifics.
But again, generally speaking, if you have a userland exploit using js jit or regex jit or whatever jit, you should have writable & executable memory.
So assuming your webkit exploit copies the binary data gotten from compiling C code from theflow to that jit executable memory area, triggers a vulnerability, takes control of userland using rop & launches execution of the exploit binary data, it should be enough to trigger the UAF then the ps4 would crash!
The next step is to add appropriate code in the C poc to take control of execution at kernel level, just like in userland but with kernel gadgets. To do that, in the poc, you will need to find a way to plant data in the kernel memory area where the UAF occurs so that the crash can be avoided & execution control taken, it will most likely require pivoting the kernel stack to a kernel planted custom krop stack.
The custom kstack will lead execution into preparing a memory area to store the payload in kernel memory, load the kernel payload from userland, then launch it & finally restore kernel execution with properly filled registers.
The vulnerability used in the exploit from theflow is found at least on 4 freebsd major revisions (9 - 12), possibly more, I received the freebsd cve mailing about it a few days ago. It is possible that he just developed the exploit on PC & ported it to ps4 with little modifications. It's one of the advantages of working on ps4 kernel, you can test many things on freebsd 9 x64 on PC, it should also be an advantage for writing a ps4 emu for pc..