I think I found it. Didn't bother to look this up until your post and can confirm by default the "Media" is not utilized. Looking into a bug report about exactly that from Feb, 2025 (a year ago from now) https://github.com/FreeTubeApp/FreeTube/issues/6786 the developer says a set of flags is needed to pass in and a user suggests following: https://github.com/FreeTubeApp/FreeTube/issues/6786#issuecomment-2832299878
flatpak run io.freetubeapp.FreeTube --enable-features=AcceleratedVideoDecodeLinuxZeroCopyGL,AcceleratedVideoDecodeLinuxGL,VaapiIgnoreDriverChecks,VaapiOnNvidiaGPUs
I can confirm this will use some "Media" activity. But comparing (unscientifically) the GPU and GFX in fdinfo, the utilization doesn't seem too different. So overall I am not sure if that is actually hardware acceleration. Also make sure FreeTube has permission for "GPU acceleration" under "Device" set with Flatseal.

