Most of the time people don't go through the effort of putting their mailservers/nameservers/etc. behind a proxy.
I'm pretty comfortable guessing that 95% of CPanel/Plesk users that use cloudflare(or another CDN) _and_ host their own mail/name-servers don't put the latter behind a proxy; and they often are on the same box as the webserver.
Edit: Which is to say that this doesn't effect someone doing it 'right', but almost everyone is sloppy(most people just don't care as they're not actively being DDOS'd).
In reality even using this to find the webserver they will eventually get wise to how you're finding the IP(likely) and swap to a new one(depending on their hosting situation) this time putting all other DNS resources 'behind proxies'.
If people don't put in the effort for the security they need, then they won't have that security. This applies to any concept and I dont see how this has anything to do with a single vendor who just provides the tools and service.
Which is obviously not the case, but that's what the marketing says.
Either you invest effort into security anyway and you don't need CloudFlare, or you don't invest effort into security and CloudFlare won't save you either. In neither case is CloudFlare the solution.
Marketing does not absolve you from proper configuration... clearly you have it out for this company for some reason.
I don't think this is a 'CloudFlare vuln' or the responsibility of CloudFlare to resolve etc...
I maintain that it is sloppy work that leaks the underlying webserver IP, but also that few people care about doing so.