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.
SuphatraMatt Wood commented
If it's tagged as "we have this!" does that mean it's now being ignored? MS Teams definitely does NOT have proper two-way interoperability. A Teams user cannot initiate a chat with a Skype for business user, making transitioning from SfB to Teams much harder.
This is still being worked upon. I hope to have more details in the next couple months.
Please know the team is actively working on this feature.