For anyone who is concerned about the level of authoritarian control companies like Amazon have over employees, you might find the book Private Government worth a read: https://press.princeton.edu/books/hardcover/9780691176512/pr...
Why would you use a work-provided chat app to discuss things the company may not like? For example, do you not expect your employer to know if you're browsing for another job over company provided Internet on company provided laptops? If they were spying on the guest Wifi, that's another thing.
In a digital company like Amazon, I can imagine that it is very hard to reach your co-workers in any other way. Most likely there will be no public list of email addresses or phone numbers, so what other options do they have if they want to bring up unionizing to their co-workers? Nowadays you cannot just catch people at the factory gate.