By the same line of reasoning, your customers could be faulted for believing you had a contingency plan.
Communicating about risks OR outages are both hard, and every company has both. I'm actually a happy (though impacted) Netlify customer. But it's completely bizarre to me to try to invalidate this customer's complaint.
[1] https://web.archive.org/web/20200303050851/https://docs.netl... (search "flattening")