alt Hacker NewsLGPL allows compiling the whole of ffmpeg into a so or lib and then dynamically linking from there for your closed source code. That's the main difference between LGPL and GPL.
But if you change or add something in building ffmpeg.so that should be GPLed.
Apparently they copied some files from ffmpeg mixed with their propitiatory code and compiled it as a whole. That's the problem here.
Copyright law defines derivative work by substantial similarity and dependence, not by technical mechanisms like linking. Technical measures such as linking is not a copyright concept.
Dynamic linking is a condition for LGPL compliance, but it is not sufficient. Dynamic linking does not automatically prevent a combined work from being a derived work.