Given the cost of laying fiber lines across the ocean and this tech (appears) to double the capacity of an existing line, why would there not be a push to get this into use, what am I missing?
This specific thing is not faster than previous results, but more compact.
Long-distance fiber lines also have amplifiers along the way, so you can't just scale them up by changing the endpoints if it doesn't match the capability of the in-line hardware.
As for it being “only more compact” not a capacity increase, for a comparable single coherent optical fiber line, are existing fibers filled to capacity due to the limits of tech, economies, physics, etc. - if physics, then I assume all fibers are at capacity, right?
The economics of it are pretty interesting. A single fiber (non-submarine) is about ¢8 a meter in raw cost, and it said that they laid out so many during the telecom bubble of the late 1990s that there are still many unused (so-called dark) fiber networks throughout the US. See for example https://www.ofsoptics.com/lighting-up-dark-fiber/