Our doc for On-Prem MMS:

http://mms.mongodb.com/help-hosted/current/faq/management/#my-alert-email-says-my-host-s-are-exposed-to-the-public-internet-what-does-that-mean

has a similar description for the alert Host is exposed to the public Internet as for the Cloud version.
In fact, On-Prem is testing that the MMS server can connect to the mongod on the monitored hosts. It is an alert that rarely makes sense in that context.

For the purpose of this bug, we should explain that we are not reporting that the host is exposed to the public internet, but simply it is visible from the MMS server.