I am having trouble understanding the fact the french or indeed any military cannot detect the signals from these boxes.
The ocean has layers of different temperature and salinity. These can act, for sound waves, in the same way a prism or mirror acts for a light wave - refraction and reflection.
Some sounds can be reflected between layers and travel across thousands of miles. Others can be emitted from just a few hundred feet below and not heard within a few miles.
Now consider a black box under 6000+ feet of water with multiple layers between it and the surface. take a submarine with a crush depth of less than a thousand feet. take a couple of submersibles with a greater crush depth, but miniscule range and no sonar.
Now consider how lucky one must be to detect the signal, and if detected, to locate the source.
I would have thought their best chance of a detection was the SOSUS chain.