However saying D dorian has nothing to do with C major (ionian in this case) is not quite what your own quote says. Of course it has something to do with C Ionian - it's the same notes! Your quote is right but you are not.
Blame the greeks - they invented it, and modes.
Interesting that the modes were named after the main greek tribes, re-invented by the Christian church in the middle ages and then replaced by the modern method of key signatures in the 17th century. So it's not worth arguing too much about the Cmaj scale or the C Ionian mode - they are the same - just a different usage. In the real world we need to communicate ideas to each other as easily as possible so we are often guilty of mixing the language of modes with the language of keys. Does it matter though, as long as we understand what is being communicated?