It's primarily the significance of the amount of time before the action (or whatever). Prior to this new study, arguments purporting to show that the relevant notion of free will was undermined by experiments such as Libet's were unsuccessful. Al Mele (FSU) wrote the definitive work on this issue in "Effective Intentions" (2010, Oxford University Press). Mele convincingly showed that the ~500ms time delay in Libet style experiments was not long enough for the skeptical argument to go through successfully. In the last part of the book Mele commented on the implications of any studies of this type (perhaps heading off arguments based on future experiments such as this one). For sake of length I won't quote the semi-formalized version of Mele's argument in the last part of the book, but rather the analogy he uses to elucidate it:

"Consider an analogy. Max struck a log with his red ax, thereby causing the log to split. If Max’s ax had been green, it would have split the log just as well. But he was under strict instructions to split wood only with red axes, and he was committed to following the instructions. If his ax had been green, he would not have used it; in fact, he would have looked for a red ax and split the log later, after he found one. In this scenario, the fact that the ax is red is causally relevant to Max’s splitting the log when he does and therefore to the actual log splitting action he performed, an action that has a specific location in time. Similarly, in the imagined experiment, the fact that at t, Sam made a conscious proximal decision to press seems to be causally relevant to his pressing when he does and therefore to the actual pressing action he performs. I should add that although we do know that, other things equal, red and green axes split wood equally well, we do not know how effective unconscious decisions are. Nor do we know whether unconscious deciding (as distinct from unconscious nonactional intention acquisition) is something that actually happens. Also, for all we know, if there are instances of unconscious deciding, they are far too rare for there to be more than a glimmer of a chance that if Sam had not made a conscious proximal decision to press at t, he would have made an unconscious one."

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It is worth pointing out that Mele is famous for remaining agnostic about free will (and certain fundamental questions surrounding it).

It's not as if they completely disregarded the existence of Libet; he is referenced many times in study and even the article posits that the research is supplementary to previous work done in the field.

I am speaking from memory, but iirc Libet's study wasn't without flaws and it was contested many times during the years.

More researchs like this are positive, as this is a far from solved field.

I had heard of this as a factoid about 30 years ago. Although I didn't know about Libet's experiments.

From the Wikipedia article -

That is to say, researchers recorded mounting brain activity related to the resultant action as many as three hundred milliseconds before subjects reported the first awareness of conscious will to act.