The Tokina 11-16mm f/2.8 AT-X Pro DX II is an APS-C lens that does not project a large enough image circle to completely cover Full Frame sensors when at its widest zoom setting. As the lens is zoomed in, the image circle does enlarge. At approximately 13mm and longer the image circle is large enough to cover a FF sensor.
Any additional vignetting you would experience when using the filters with this lens on a FF camera would be in the areas at the edge of the sensor that are just inside of the edges of the image circle projected by this lens.
Since I have a Tokina 11-16mm lens the P holder would introduce vignetting for lenses wider than 20mm (Cokin site) while the Z serie ranges from 49 to 96mm.
You seem to be conflating angle of view, expressed in focal lengths using millimeters, with lens thread diameters, also expressed in millimeters.
The Size M (previously 'P' series) is smaller than the Size L (previously 'Z-Pro' series) which uses square or rectangular filters that are 100mm wide on the short edge. The Size M (P) series are only 84mm wide. With the Size L (Z-Pro) Series, wider angle lenses of the same diameter can be used than with the Size M (P) series. Exactly how wide depends on several factors:
- The diameter of the lens' front element
- The diameter of the lens' filter threads and the difference to the front element diameter
- The distance the filter threads are in front of the edges of the front element
- The thickness of the filter holder
Cokin is fairly conservative with the focal lengths they more or less "guarantee" will work with different sized filters. This allows for scenarios with lenses that have lens elements almost as wide as the filter threads and set back from the threads by a relatively large margin. They also measure vignetting with the square filter and holder rotated 45° with respect to the long axis of the camera's sensor (because sometimes one may want to orient a graduated filter at that most restrictive angle). If the filter and holder is square with the camera's sensor, a wider angle lens can be used before vignetting is demonstrated.
With Size L (Z-Pro) filters, one can use focal lengths shorter than 20mm on a full frame camera with most wide angle lenses before vignetting is demonstrated. The Size L (Z-Pro) series officially allows for a somewhat nebulous sounding "20mm and wider" focal lengths with the 35mm/FF format. They also say that "between 16 and 20mm, some vignetting can occur." The even larger Size XL (X-Pro) series with 130mm wide filters allows lenses down to 14mm focal lengths in 35mm/FF format without vignetting (when the filter and holder are rotated 45° with respect to the sensor).
In practice, depending on the way a lens is made, one can get good results wider than the minimums stated by Cokin, particularly when the filter and holder are not rotated at an angle with respect to the wide axis of the sensor. Since the Tokina 11-16mm f/2.8 AT-X DX lens has a fairly flat front element (compared to the bulbous fronts of lenses such as Canon's EF 11-24mm f/4 L) that isn't recessed very far behind the filter threads (compared to Nikon's 12-24mm f/4 DX), I'd expect that you could probably expect none to minimal vignetting at 16mm that would get increasingly worse as you open up with the Size L (Z-Pro) filters. You should be able to get all the way down to at least 14mm with the Size XL (X-Pro) filters, and probably even to 13mm where the lens' image circle size causes vignetting on a FF sensor when the filter and holder are squared with the sensor.
The "49mm to 96mm" number at the link to a reseller of Cokin filters you included in the question is in reference to filter thread sizes on lenses, not the focal length of lenses. Adapter rings are available to attach a Size L (Z-Pro) filter holder to lenses with filter threads as small as 49mm and as large as 96mm.
For more basic information about the Cokin filter system than what is provided by the reseller site to which you linked, you can check out:
The Cokin System: A Beginners Guide at Cokin USA's marketing site
The Cokin Creative Filter System at B&H Explora
Cokin's Size Guide at Cokin.com