This was one of my favorite cupertinos, because, for us readers, imagining that something could be defiantly true was kind of an interesting stretch. No, the writer usually meant definitely, and by virtue of typing an a in there somewhere, was diverted into a different word by a computer spell-check which takes vowels to be very important. I think the computer spell-check figures that native speakers usually know the vowels and are more likely stuck on the consonants, i.e. whether to have two of them. Internationals are very likely, or at least more likely, stuck on the vowels. Look at the trouble they have finding and spelling words like obviously.
One thing I'd like to say about this one, is that it's pretty limited in its time frame. Just as cupertino itself was only a cupertino during the early days of spell-check, defiantly as a cupertino for definitely occupies a limited time frame. By that I mean that the makers of spell-check noticed that it was a problem, took steps to ensure that those who intended definitely got it, and made spell-check just a little more sensitive so they could be sure people got what they wanted. In other words, it was a success story for spell-check, more or less. It's not over yet; one still sees defiantly. Not everyone got the 2014 update, or whichever one it was that improved it. Not everyone is thoroughly updated, thoroughly all the time. And that's one of the variables one has to deal with.