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"Attacker has ROP and shouldn't be able to make arbitrary syscalls".

Seems mildly useful if you have a really flexible syscall you can't forbid (ioctl, say) but which you only use for a specific narrow purpose.

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If they can ROP they can jump to a syscall instruction with controlled arguments

The point of pinsyscall is that they have to jump to the single entry point for that syscall, rather than any of the ~200+ syscall instructions littering the address space. ALSR makes finding an entry point difficult, but that's easier if you only need to find any syscall instruction, rather than the specific one for the syscall you're invoking. The rationale is explained here: https://undeadly.org/cgi?action=article;sid=20230222064027

I’ve already expressed my opinion on this particular mitigation elsewhere and I’m pretty sure you’ve read it so I won’t go into it again here. I was more interested in the scheme that the other comment was constructing.

The point of what I spelled out above is that they can jump to the instruction but the kernel will kill the program if they don't go through the function up to that point. That allows you to restrict the arguments to the syscall at the point of call.

Ah, so it’s like a poor man’s BTI

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