I have the Playbar and TV 4k. Since the Siri Remote also uses IR Signals and is placed behind my TV I never have a direct line of sight to the TV and have yet to experience any issue with controlling it or the Volume. If I leave the living room I simply point in the general direction of the Doorway and it has always worked as I intended.
The Sonos Playbar only has optical input which cannot pass volume control commands (well, a device could lower volume by dividing down the actual sample numbers, but this would result in increasingly bad compression at anything less than full volume - I haven't heard of any device actually doing this).
Line of sight to the Apple TV is irrelevant, since it doesn't
normally receive IR signals - it can be taught to watch the IR signals from a 3rd-party remote, but this isn't used by the Siri Remote - the Siri Remote can send commands over IR, but it only does so to TVs or soundbars, if you don't have CEC available. The connection between the Apple TV and the Siri Remote is always bluetooth.
Yes, "line of sight" was an oversimplification, sorry. IR signals will bounce
some - in your case, enough to work. In general, I wouldn't depend on them to bounce enough to describe it to random people as always working - depends on your specific use case. So,
in general, it's largely line of sight. With the bluetooth transmission of the Siri Remote, and a CEC connection from the AppleTV to the TV, I don't have to aim at a doorway or
anywhere - if I'm a room away and around the corner in the kitchen, crouched down looking at something in the fridge, with several corners and the fridge door between me and the TV, I can still control the volume (or pause) using the Siri Remote. It's not that I have to have this as a feature, it's that once I've gotten used to it, I'd be annoyed to lose it.
Fun fact: in the latest iteration of the Siri Remote, at least, it's fairly evident that if the Siri Remote senses that the Apple TV is off (it can't establish a connection), it will revert temporarily to sending volume control commands via IR directly to the TV, presumably based upon knowledge of the TV model it gleaned from previous CEC connections (I've never set up the Siri Remote for IR control of the TV). If I'm watching something but my Apple TV is off - say I'm using the PS5 or on rare occasions watching over-the-air TV broadcasts - if I suddenly need to change the volume, I'll often grab the Siri Remote to do it, since I've found it to
always work, it's always nearby, and it's easy to get to the right buttons quickly since there are so few.