How does the medic work in Pandemic - In the lab once a disease is cured?
Yes, your interpretation is correct.

From the rules for Pandemic In the lab:


  A cube removed from the board by Events, the Containment Specialist’s
  ability, the Medic’s ability to automatically remove cured disease
  cubes, etc. may not be placed in a sample dish.


From the rules for Pandemic: 


  The Medic removes all cubes, not 1, of the same color when doing the
  Treat Disease action. If a disease has been cured , he automatically
  removes all cubes of that color from a city, simply by entering it or
  being there. This does not take an action


I could find nowhere in the rules that said the medic's ability was optional.