What we noticed about the battery is that its model number matches that from the FCC certification of the Galaxy S8 Plus almost completely, so we're inclined to believe the power pack is what will actually be used in Samsung's flagship this year.
What the leak doesn't revel is the actual manufacturer of the cell. Samsung has been rumored to look into different battery suppliers than those for the Galaxy Note 7. And if rumors are true, this power pack could've come from a number of different companies.
However, until we see the Galaxy S8 Plus for ourselves, all these leaks and rumors will remain unconfirmed.