The fact is that Fil-C allows capability and pointer writes to tear. That is, when thread 1 writes pointer P2 to a memory location previously holding P1, thread 2 can observe, briefly, the pointer P2 combined with the capability for P1 (or vice versa, the capability for P2 coupled to the pointer bits for P1).
Because thread 2 can observe a mismatch between a pointer and its capability, an attacker controlled index into P2 from thread 2 can access memory of an object other than the one to which P2 points.
The mismatch of pointer and capability breaks memory safety: an attacker can break the abstraction of pointers-as-handles and do nefarious things with pointers viewed instead as locations in RAM.
On one hand, this break is minor and doesn't appear when memory access is correctly synchronized. Fil-C is plenty useful even if this corner case is unsafe.
On the other hand, the Fil-C as author's reaction to discourse about this corner case makes me hesitant to use his system at all. He claims Java has the same problem. It does not. He claims it's not a memory safety violation because thread 1 could previously have seen P1 and its capability and therefore accessed any memory P1's capability allowed. That's correct but irrelevant: thread 2 has P2 and it's paired with the wrong capability. Kaboom.
The guy is technically talented, but he presents himself as Prometheus bringing the fire of memory safety to C-kind. He doesn't acknowledge corner cases like the one I've described. Nor does he acknowledge practical realities like the inevitability of some kind of unsafe escape hatch (e.g. for writing a debugger). He says such things are unnecessary because he's wrapped every system call and added code to enforce his memory model's invariants around it. Okay, is it possible to do that in the context of process_vm_writev?
I hope, sincerely, the author is able to shift perspectives and acknowledge the limitations of his genuinely useful technology. The more he presents it as a panacea, the less I want to use it.