That's not it. The LGPL doesn't require dynamic linking, just that any distributed artifacts be able to be used with derived versions of the LGPL code. Distributing buildable source under Apache 2.0 would surely qualify too.
The problem here isn't a technical violation of the LGPL, it's that Rockchip doesn't own the copyright to FFMPEG and simply doesn't have the legal authority to release it under any license other than the LGPL. What they should have done is put their modified FFMPEG code into a forked project, clearly label it with an LGPL LICENSE file, and link against that.
Could there have been other / better moves with sending reminders, now the OSS community loses the OSS code of IloveRockchip, and FFmpeg wins practically nothing, except recognition on a single file (that devs from Rockchip actually publicly acknowledged, though in a clumsy way) but loses in reputation and loses a commercial fork (and potential partner).
"In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License."
They should be covered as an aggregation, provided the LGPL was intact.
How does
"Distributing buildable source under Apache 2.0 would surely qualify too"
reconcile with
"doesn't own the copyright to FFMPEG and simply doesn't have the legal authority to release it under any license other than the LGPL"