Why would anyone not think that putting a document in a public channel would result in people who can join that channel having access to it? Isn't that the whole point?
The mismatch makes sense to programmers and IT types who have internalised the “computers are stupider than we make them look” way of thinking… but most normal non computer professionals are probably just thinking “I am sharing this with these people right now.
To be brutally honest people need to just stop and apply some basic critical thinking to their actions instead of expecting magic.