This is wrong. The text you refer to talk about linking, and yes you can use any terms for an application dynamically linking to a LGPL library. So if Google shipped Chromium which was dynamically linked to use the system ffmpeg install on Linux distributions then that of course is fine.
But Google is not shipping just a dynamically linked application that is meant to use a 'system' install of ffmpeg. They are shipping ffmpeg themselves. And ffmpeg is LGPL licensed, period. You are misunderstanding how source code licenses work if you think that text in the LGPL gives you the right to re-license code under the LGPL.
And the patents licensed for H264 support are for the code in ffmpeg, not for the BSD licensed code in Chrome. So while it would be fine for Google to license a patent covering bookmarks or something like that in their BSD licensed UI, they can't pretend the codec patent license is for the UI, as what the patent texts for H264 related patents describes functionality is found in ffmpeg code, not in the UI code.