It's not really extending it by 17 years though since they don't have data for the full 17 years. They have snapshots from 17 years prior to the start of actual data collection.
I haven't actually poured through the data set linked in the article, but skimming it gives the impression that it includes a variety of images from 1964-70, which seems to me like a lot more than just a snapshot. It even mentions in the article that they were able to see multiple years of polar ice.
That would still be a snapshot. There is no monthly data, or even yearly on the same date by the sounds of things, nor does it backfill up to 1979(when we started actually collecting data)