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.
SuphatraKris Cremers commented
David Johnson's analysis of the issue is very much on point. This is a real problem for someone who solely uses Teams for communications to people who use both platforms. Please escalate and fix this issue!