Try to assume the best in people and their actions and your life will likely be better for it.
That said, the company owns that Slack instance. They can do whatever they want with it.
Now, it might be unethical for someone else to post as another person, but I'm pretty sure it wouldn't be illegal.
Slack does make it difficult for anyone to remove a post made by someone else. That is intentional. But if you can log into their account as them, then I guess they can do whatever they want as the person in question.
Disclaimers: IANAL, YMMV, etc....
People have to remember, these are business accounts, not personal accounts and all their content belong to the business --which is why one should avoid using business accounts of any type for personal use.
This is solid advice. After 20 years of (trying to) implementing it, it's made for much better professional and personal relationships.
The job is over, with very little to be gained dwelling on trivialities.