Island users can't communicate with externals using Skype client

I have a customer who is on S4B on prem (all users homed on prem) and some users also have the Teams client (https://docs.microsoft.com/en-us/SkypeForBusiness/hybrid/plan-hybrid-connectivity#implications-of-the-retirement-of-skype-for-business-online).

Those users with Teams can initiate a Skype chat with external users (federation) but the recipients (in TeamsOnly) cannot reply to that conversation. Instead they get this message

and by clicking the link it takes them to a new native Teams chat where they can write to the user in Island 

The user in Island receives the message in the Teams client (which shouldn't happen) but they cannot reply in that chat. they the get message "Due to org policy changes, this chat is no longer available. Continue with Skype for Business"

I have read a few sources in MS docs where it says that users in Island mode communicating with external (federation) should receive all messages in S4B client. 

Routing for federated chats and calls differs from in-tenant routing in that Islands users will always receive a federated communication in Skype for Business. This is because the federated partner may not yet be using Teams. Routing to Skype for Business for any islands mode recipient ensures messages will always be received. Routing to Teams could potentially result in missed communication if the intended recipient does not use Teams.

https://docs.microsoft.com/en-us/microsoftteams/coexistence-chat-calls-presence#federated-routing-for-new-chats-or-calls