While you are not lying, and mailing lists should be avoided if you want to share secrets, most of the times you need a mailing lists not to do that, but to simplify communications.

At least in the global south, most of radical activists groups have strong "no-internet policies" for any type of secret, and no cellphones ones for their work. They have learned with their own history what they can or can't do, learned how to deal with IRL infiltration, and even learned how to communicate without any kind of contact or even agreements between groups. To survive and act against dictatorships or invading armies is not easy, they had to be smart.

But still, because travelling is expensive and networking today is a need for some of those groups or collectives, they can communicate with each other talking about their resolutions or activities, which are not secret (as I already said, it's assumed there can be a IRL infiltrate) but they are also not public.

You can see SMTP and mailing lists as a huge security risk, and they are, it's just not very common to see people that assume the opposite around here.