alt Hacker NewsI get where you're coming from, but I think the part you're missing is that caching is not optional (per the "with the cache disabled" comment). If you're running a service like theirs, you need to do caching to some degree.
My understanding of their implementation is that it does caching on a per-worker basis with lower TTLs, so it balances caching with accuracy, but doesn't "fix" visibility to the problem either. However, it narrows the window such that their method of monitoring will very likely expose a pulled domain sooner.
Caching is always optional. If you want sophisticated control over caching, or just no caching at all, you can do recursive lookups directly. Some existing DNS libraries can do recursive lookups directly, including libunbound. Ideally you would do caching of TLD nameserver addresses so you're not hammering the root servers for each query. But otherwise I don't think it's necessarily a big deal for a monitoring service to query the .de nameservers every time it checks example.de. If you had a monitor check every 5 minutes for example.de, it's only a 12x load factor between querying .de directly every check versus once per hour. Unless you're AWS Route53, that's not much of a difference relative to the traffic TLD servers handle even if you were checking tens of thousands of domains. Clamping your cache at 1 hour is probably already a much greater load factor. The .de nameservers return a TTL of 1 day for the example.de NS RR sets, so that's already a 24x factor right there. At that point you're clearly giving the .de nameservers the middle finger and relying on your own judgement of what it means to play nice.