Passwords (when you use a password manager) are as secure as your computer.
If your computer is compromised, you are out of luck anyhow. As the attacker then can just use your logged in browser or app to post whatever they like.
There are many ways to compromise passwords other than achieving access to the user's computer, such as phishing and password reuse. Use of a password manager mitigate these attacks a bit, but not completely.
If your computer is compromised, you are out of luck anyhow. As the attacker then can just use your logged in browser or app to post whatever they like.