The article is a little weird. This is my interpretation, need to look for other sources.
President says that the ambassador is "taken hostage, literally".
Military in Niger wants him out of the country, but French government says that his work is too valuable, he must stay.
Military says " OK, but he can't leave the embassy, Persona Non Grata".
Technically the ambassador has either choice of leaving the country or staying inside the embassy. Food is not delivered anymore, leaving him with military rations to eat.
Scratch my previous summary. Yours wins the cake.