The article seems confused. First:
"It couldn't have formed in place because you can't form a planet inside a star. It couldn't have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma," explains Sasselov.
Kepler-78b is a doomed world. Gravitational tides will draw it even closer to its star. Eventually it will move so close that the star's gravity will rip the world apart. Theorists predict that Kepler-78b will vanish within three billion years.
It seems we're simply observing it as it slowly spirals into its star, but before it's arrived. What's enigmatic - the happenstance of the timing of the observation?
But perhaps something was lost or added in the press release
for some pizazz. Here
is another article with a bit more depth on the process of investigation, but no mention of the lack of knowing how it formed.