@Christopher Hoard 


I am not sure i fully understood the setting we are required to do, but i could understand the bottom line that is written clearly in your great blog that :


"DLP for external chat sessions will only work if both the sender and the receiver are in Teams Only mode and using Microsoft Teams native federation."


In other words, if the other side is not Teams only or is Skype for Business - content will not be inspected and sensitive data can  be shared. Is this a correct understanding?