Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
Originally the battle spectator list on smogtours was invisible to everyone, till an update in 2020 changed to make that public. There was an error in how it was implemented so the spectator list only shows who clicked on the battle after you, instead of showing everyone watching the game currently (like how it does on main). Luigi clarified to me last year (while he was still TD) that it should've worked as the latter.
If you were looking a more technical clarification, Kris would probably know.
There’s nothing special about spectators, they’re just broken. They should work like main does. Only different thing is battles are forced public and you can hide the replay.