Hi
Probably because I read about someone using it! The margins between all the threads are quite fine. The cap thread reduces the diameter of the barrel tenon to 11.2mm. The nib thread in the barrel gives an inner diameter at the maximum thread depth of 9.75mm. This leaves a wall thickness between the threads of 1.45mm divided by 2 if I've understood taps and dies correctly. I realise that sounds thin but in practice it hasn't caused any issues.
With the nib section itself the wall is even thinner. The tenon is turned to about 8.8mm and the 9 X .75 die is used on it. This gives a diameter of 8.25mm. The hole for the refill is 6.4mm. This gives a maximum wall thickness between the inner wall and the bottom of the thread of 1.75mm divided by 2.
Overall the threads part of the barrel has a thickness of about 1.8mm and the nib section one of 1.3mm. My thinking is that it's better to have he thicker tenon on the barrel as this gets more use than the nib section thread.
Here's a shot of the two tenons....
