The article in the OP states that AF asks obese pax to book a second seat only when the plane is fully booked. Hence it's reasonable to assume that the outbound flight wasn't fully booked and AF CDG could accomodate the obese pax with an empty middle seat.

Btw, this reminds me of WN's policy. They also ask POS to buy a second seat when flights are "fully booked". Of course, this is a bit strange, considering WN is single class (no possibility for up-ups) and empty seats don't magically appear on "fully booked" flights by simply paying for them. So I guess the term "fully booked" refers to the booking situation a few hours before the flight's departure (typically during check-in) and doesn't count in any no-shows.