I tried just now in the advanced sample react app, and I can confirm this:
getMeetingParticipants() is only allowed if you are the meeting owner (the one whose account the meeting belongs to).
Being co-host or assigned as host does not matter. You can only see get the meeting participants list if you’re the meeting owner, and not if you are connected to someone else’s meeting, even if you’ve been appointed as host or co-host.
I would like for this to change, to be in line with the other SDKs. The getAttendeeslist() method works even for guests in the Web Meeting SDK, and it’s great! This is a show-stopper feature for me.