I think you should read the text a bit more closely. A 700km impactor would be required to completely boil the oceans into vapor, but the article mentions that a much more insanely huge impactor of between 2000 and 2,700km would be needed to genuinely sterilize the earth because it entirely melts the crust.
However, (and here I speak from reading many other sources on this subject), even something the size of that latter object (basically something the size of a planet like Pluto) might not be enough.
Other studies have indicated that it might be exceptionally hard for heat to transfer enough through the vast mass of the earth's upper mantle and crust to actually melt it uniformly even if hit by something like the object that may have formed the moon.
This impactor was also supposed to be roughly 2,500km across and some of the theoretical concepts around it argue that even in that absolutely cataclysmic scenario, at least a part of the crust remains intact and relatively cool below a depth of several hundred meters.
If such a thing were to be true in the context of modern earth, which teems with life in every single possible remotely habitable nook and cranny and crevasse, then even a colossal impact by a 2,700km planet-type object wouldn't sterilize the Earth of life.
If only part of the crust remained intact as described above, then after hundreds of thousands or a few million years, as the earth's partially molten surface cools, and an atmosphere reforms, water that also reforms from vapor would rain down to the surface again in a vast global storm like something out of the bible. As this happens and the oceans refill, the tiny organisms concealed during all that time inside the deep cracks hundreds of meters below the surface of that surviving part of our world's crust would then recolonize the surface eventually and cause life to flourish again.
The key things in this scenario are that for one, at least one small part of the crust remains intact and cool, at least below a certain depth, and secondly, that water vapor eventually re-condenses into rain.
One might think that water would be vaporized right into separation of its hydrogen and oxygen atoms by the heat of such impacts, but mostly no. For that to happen the water on Earth would need to be subjected to at least something like 3,000 degrees Celsius of sustained heat, and even a planet-sized impact wouldn't generate such temperatures over most of the world.
