@El'endia Starman's comment is correct. One of the common assumptions of the young-earth position is that the dating methods that give us billions or millions of years of history are flawed. I could give hundreds links to support the statement that this is a well-accepted assumption among YEC-ists, but I'll limit myself.
We'd also counter with "If the dating methods are so accurate, how is it the coelacanth stayed un-evolved for millions of years?" and a whole host of other similar questions.
I'd also point out that the article makes no mention of how the author came to the conclusion that the thing is 100 million years old. It just states it as if it were undisputed fact with no evidence to back it up, which wouldn't even fly here, as there's no supporting references. We don't even need to bring up the dating methods for this article. We could just ask where that figure comes from. (Then we'd need to debunk the dating methods.)
The answer could and probably should end with the above, but in the interest of stemming the all-but-inevitable debate in comments, I'd like to add...
None of which proves anything, either way. All such questions do is lead to endless pointless debate.
Neither side is likely to throw aside our presuppositions to give credit to the opposing side. To the atheist, the idea of allowing for supernatural explanations is incomprehensible. For the YEC Creationist, there's nothing less than the entire foundation for Christianity at stake. And of course, in between those two polar opposite views are hundreds of shades of differences, concern, and apathy.
And note, I did not state that evolution is true or false, I was very careful to stick to "How do YEC defend" not "Is the YEC view true", despite my well-known opinions on the matter. This isn't the place to debate which position on origins is true. I answered only because this asked specifically how a certain group within Christianity typically responds to the given statement. "How they respond" is answerable objectively. ;-).