In my understanding the program can work correctly in normal use.

It is buggy because it fails to check that s->idx is in bounds, but that isn't problem if non-adversarial use of s->idx is in bounds (for example, if the program is a server with an accompanying client and s->idx is always in bounds when coming from the unmodified client).

It is also potentially buggy because it doesn't use atomic pointers despite comcurrent use, but I think non-atomic pointers work reliably on most compiler/arch combinations, so this is commonplace in C code.

A somewhat related issue if that since Fil-C capabilities currently are only at the object level, such an out-of-bounds access can access other parts of the object (e.g. an out-of-bounds access in an array contained in an array element can overwrite other either of the outer array)

It is true though that this doesn't give arbitrary access to any memory, just to the whole object referred to by any capability write that the read may map to, with pointer value checks being unrelated to the accessed object.