That assessment kind of leaves out that much of their Muslim problem was created by forming colonies in the Muslim world.
If you take countries in Africa and the Middle East into your empire, what sort of new citizens are you getting? If you later disband the empire, well...aren’t those same citizens who came to the home country (and their children) still citizens?
Yeah, France is getting Muslims sneaking in from abroad. But it would be a trickle if there were not a well established base of French Muslims to set them up when they arrive.
All of France thought this was a great idea starting in the seventeenth century.
The French didn’t grant automatic citizenship to natives of former colonies after decolonization. The British did sort of, they granted the right to British passport to natives. The French didn’t and certainly didn’t to the Algerians after the bitter war they fought for independence.
The immigration from North Africa has been a result of the need for workers; they’ve allowed them in to work on work permits. Then of course, it all spun out of control.