Earlier quoted context omitted.
Unless you know what the DNS request is "supposed to return", you can't know that just getting any DNS response indicates that you have full Internet connectivity.
They control the record contents just as they would some text in a file on some server, and could be checked in a similar manner.
It still wouldn’t find http filtering, but it would work better than I initially gave it credit. (I still doubt it would give a contextually correct answer for an airplane wifi connection [where DNS May very well work but few other services do if not paid].)