Secure sandboxing tends to mean opportunities to make unrestricted copies.
It's 'safe to assume' it's not. It's emphatically not safe to assume any mitigation is perfect.
Why would that be safe to assume? If that were a reasonable assumption, you could just as well assume that it's safe to run ffmpeg.
A manually run ffmpeg on the command line does nothing to restrict its privileges, and its security model has very little interest in doing so, while browsers very much have.
And get hardware acceleration working...