First of all like I mentioned this is standard procedure for these kinds of things. Second of all I also found a paper that shows that they can be asymptomatic carriers of H7N1 which is highly pathogenic variant. Attached below but here’s the conclusion. Apparently they’re a popular farm animals in places like South Africa so there’s a bunch of literature on them if you want to see more.
I also checked the WOAH manual and they also say that vaccination isn’t enough on it’s own and can be at best used as an accessory to culling. Apparently the HPAI viruses (which can come into existence by either a H5 or H7 variant mutation, which are common in retroviruses like the influenza) are largely unaffected by the vaccine.
Source (archive doesn’t work for this website).
So to summarize there’s a high probability that the surviving ostriches are either carriers or have a low severity strain that can at any moment mutate into a problem.