Where do you see that 17c info? I am on the Veloflex site now and I can't find a limit. Or are you referring to the ETRTO guidelines published a while ago? These link below shows the image Mavic published:
There, it shows a 25c tire suggested to fit into an 18c max rim.
Is this what you mean by 17c max rim internal width? Or is it specific from Veloflex?
ETRTO is on the box. Minimum is 13mm and maximum 17mm (inner rim width). If you mount any non-cotton tire (stiff vulcanized 120-180 TPI), you can safely ignore ETRTO, mostly due to pre-shape of a tire. In other word, you can mount 25mm Conti 4000S II tire on very wide 23mm (inner width) rim and ride it for years! In contrast, cotton tires (supple unvulcanized 280-360 TPI) are not pre-shaped and very soft sidewall hence ETRTO guidelines should not be ignored.