Does excluding other Messaging agents skip people who are only enrolled?

No. Exclude leads sent a connection request or message from other Messaging agents skips a person who was sent a connection request, a direct message, or an InMail from another message flow. A person who is only enrolled on that other flow, and still waiting for a first action, can still be enrolled here. The other flow being inactive does not change this.

Use it when a shared list moves onto new sender flows and people who have been waiting should still be enrolled. Exclude leads contacted by the same sender from other Messaging agents still skips someone already enrolled on another flow with that sender, even when no connection request or message has gone out.