From the details (in an image, some in fine print on the image) on the offer: that's if you migrate at least 1PB into Google Cloud Nearline in the first 3 months, and maintain at least 1PB in Google Clound Nearline for at least 12 months.

And its only 1 month free at that level of commitment; anything more than 1 month requires a migration commitment to more than the 1PB.

EDIT: At the time this comment was written, the thread title referred to the 100PB free for up to 6 months offer, not the fact that Nearline was now in GA. While the content of the comment is still true, the title that is being clarified by the comment is no longer present. (Similar things seem to be true of a number of the other top level comments.)