I did have a quick look before posting and I thought I saw that bar had a hdmi input (for the pi) and then an output with ARC for the TV. I thought that would work but I admit it's been a while since I tinkered with CEC. But as others have said, it is flakey. I was subscribed to the libcec issue tracker for a while and it was relentless with per-device issues. I had a mostly good experience with a Sony TV but it seems to be pot luck.

Relatedly, does a £300 soundbar in 2020 still only come with one HDMI input!?