The Slack API documentation specifies that user_id is not globally unique here, and this post seems to state that channel_id isn't necessarily unique either.
Now my question is about the chat.postMessage API endpoint. It takes a channel_id or user_id for the channel, but has no way to specify a team. So can we as the caller still assume that those are effectively unique? The post above is over a year old, and I haven't found any updates, so I don't know if that assumption still holds.
Yes, the team/workspace is implicit in the token you're using to execute the chat.postMessage request. All specified channel IDs and user IDs will be considered within the domain of that implicit team.