"Considered" by whom?
The French government still considers them to be diplomatic facilities and so governed by treaties and international norms that such buildings are inviolable. The French government has said, "The French army is ready to respond to any renewed tension that would target French military and diplomatic facilities in Niger."
Moreover in their letter revoking the Ambassador's status the Military Junta wrote, "[He] no longer enjoys the privileges and immunities attached to his status as a member of the embassy's diplomatic staff." This would seem to imply that the Junta recognise the embassy, just not the ambassador. (Source for my quotes)
That could change. Your concern with "legal access" seems misplaced — The junta is not overly concerned with what is "legal", after all, the coup wasn't "legal" either. But there is a strong element of posturing here. What would be the benefits Niger would gain by storming the embassy weighed against the costs.
Normally an ambassador does leave when told to by the host country. The issue here is, who is the legitimate government? France doesn't recognise the Junta de jure, so they can say with some justification that no legal request has been made for the ambassador to leave.