"as recently as 3.4 billion years ago"
Don't hold your breath.
As always, this paer needs to go through the post-publication scientific process where other scientists test the evidence and see if it is really as strong as the authors suggest.
Shorelines on mars have been claimed for decades (over a century, if you include some of the more extreme Lowellian visions). All have been disproved, or severely questioned, so far...