He is in the equivalent of the VIP wing of the only prison in Paris Intramuros (within the city walls), the Prison de la Santé. https://en.wikipedia.org/wiki/La_Sant%C3%A9_Prison The prison has three main sections: the VIP wing, the night-time incarceration wing (sentence adjustment), and the high-security wing.

As the most serious cases at the national level are often tried in Paris, the high-security wing is filled with drug traffickers, murderers and terrorists, at least for the duration of the proceedings, which can take years in France.

Sarkozy is in the VIP wing with two bodyguards nearby. These are hardly the conditions one would imagine for isolation.