Cool, but if it becomes popular enough to drive liquidity, wouldn't it be vulnerable to a 15% attack?

15% is still a lot for such a large network, of course.

So the article says this new method of calculating price is based on ~15% of crypto transactions correlating to a round number in some kind of fiat

You can use educated guessing to find that round number, as you have so much data pointing to it

But that's only 15% of all transactions; if there was money on the line, you could submit your own transactions to game this system for your own benefit. You only need to beat that 15% or so of transactions the article quoted.