That is correct. Be careful with this, as it applies to all users including admins. If an admin logs out, they won’t be able to log in again to disable the read only mode, and can potentially be locked outside the forum.
If this is related to your hosted site, feel free to reach out to us if you need help.
If you only want to prevent people from posting while they still need to be able to log in, and if you’re capable of installing plugins, you might want to take a look at our prevent-posting plugin: GitHub - communiteq/discourse-prevent-posting . You can choose whether you want to allow PM’s or not, depending on your use case.