I thought the same thing myself, but looked into the issue again when we started setting disable mailing list mode to true by default. Unless I’m missing something, the Discourse code prevents mailing list emails from being sent when disable mailing list mode is set to true. That happens here: discourse/app/jobs/regular/notify_mailing_list_subscribers.rb at main · discourse/discourse · GitHub.
I have also tested this out on a few sites and see that mailing list emails are not sent when disable mailing list mode is set to true.
Assuming that what I wrote above is correct, that setting’s copy needs to be updated. The intended behaviour of the setting is to disable mailing list mode for the whole site.
The reason that re-enabling mailing list mode by deselecting the setting works is because when disable mailing list mode is set to true it doesn’t override the user options of any users who had previously enabled mailing list mode for their account. If mailing list mode is re-enabled, the user option that was previously selected will be respected.
If you are finding that mailing list emails are still being sent when disable mailing list mode is set to true, please let us know.