It looks like some people in the thread below have figured it out — we actually have this. You can start a chat both ways; the system will automatically recognize if this person is on Skype for Business or on Teams.
Thanks and let me know if you have any questions.
An error occurred while saving the commentEvan Basse commented
We tested Teams, without real integration we abandoned the migration. No idea why this is marked "We have this." You need to be able to easily start a chat or call with S4B user no matter if they have ever signed into teams.