For either of those setups to be possible, high latency is basically already required. Furthermore, if you can force a (30, 70) (70, 30) situation to happen repeatedly, then that already means that you can prevent justification from happening in two consecutive blocks, which means you can prevent finality indefinitely. FFG never claimed to give guarantees of ability to finalize under such conditions.
That said, it is a very good point that the three-round setup shows that no fork choice rule can prevent scenarios where the attacker can justify the non-canonical chain at any time, as one can construct scenarios where the attacker can justify either chain at any time.
It’s certainly an argument in favor of CBC being the right thing long term!