A proprietary newsletter software (MailChimp) is being used for no obvious reason. What criteria made MailChimp win over free software competitors like phpList? This should be documented at https://meta.wikimedia.org/wiki/FLOSS-Exchange
If this is a task about running proprietary software server-side, they are related, though I'm not sure it actually matters regardless. Even if it's all hosted externally... that would actually be worse.
The CI team and Wikimedia Operations team don't allow use of non-free software on our servers (with the exception of some firmware/router stuff where there really is no free alternative).
So using an external non-free service is different practically and technically, but yes, in principle it's no different.
I think the most productive way forward is to do what @Nemo_bis asked, and document the reasons for why a non-free mailing list software was picked, so we can work on improving the free software solutions to meet our needs, and eventually switch. I'll update the task title.