You’re right, there’s no groupcast or binding, maybe the server supports it like bindings (which is more a feature of the devices actually) but the user interface does not expose those features.
(Edit: Marcel recently said no support for group management yet, it’s on the to-do list)
Baring that, is command batching from 1.3 implemented? How can I tell if this is being used? As I understand it, this would allow multiple single device turn-on commands to be in the same message
Command batching for popcorn is only useful for non-Matter devices that are bridged. The batches are commands for the bridge that include actions for multiple endpoints (the devices that are bridged). If the devices are Matter, the only way to address them all at once is groupcast.
Scenes in Matter 1.3 use extensive use of groups and I believe that would be the natural mechanism to avoid popcorn effect, you can create group scenes, recall group scenes, etc. However, in smart home platforms with no support for groupcast, Matter scenes are going to be pretty much useless and “compatible with Matter 1.3” will become meaningless. What’s the point of including the definitions of Matter 1.3 or Matter 1.4 clusters if they refer to group IDs that you can’t send commands to?