no it isn`t

without sig file

flatpak install HandBrake.Plugin.IntelMediaSDK-1.8.2-x86_64.flatpak
ID Branch Op Remote Download

1. [✗] fr.handbrake.ghb.Plugin.IntelMediaSDK 1 i flathub 0 bytes
   Error: GPG verification enabled, but no signatures found (use gpg-verify=false in remote config to disable)
   error: Failed to install bundle fr.handbrake.ghb.Plugin.IntelMediaSDK: GPG verification enabled, but no signatures found (use gpg-verify=false in remote config to disable)

with sig file

flatpak --gpg-file=HandBrake.Plugin.IntelMediaSDK-1.8.2-x86_64.flatpak.sig install HandBrake.Plugin.IntelMediaSDK-1.8.2-x86_64.flatpak
Segmentation fault