Your right! But the outbox pattern is good enough for a lot of purposes. The outbox pattern works if the only reason the write to the 2nd system can fail is because of transient issues. It will keep trying until the system is back up.

If the 2nd system write can fail for non-transient reasons, the outbox pattern doesn’t work and you need either 2 phase commit or a distributed saga.

I wrote about this here a few years ago.

https://linuxblog.io/the-two-generals-problem/