I'm not using the service either, but I noticed this comment [1]. It's not the first time that a DNS server has been DDoS-ed, so it has been discussed before (e.g. [2]). At minimum, I would expect a company that exists for scenarios like this to have more than one DNS server. Staying up when half of existing DNS servers are down is a new problem that no one has faced yet, but this is an old, solved one.

[1] https://news.ycombinator.com/item?id=12759653

[2] https://www.tune.com/blog/importance-dns-redundancy/