Rule R12, including its "red box", reads as follows:
<R12> Certain non-VEX screws, nuts, and washers are allowed. Robots may use any commercially available #4, #6, #8, M3, M3.5, or M4 screw up to 2” (50.8mm) long (nominal), and any commercially available nut, washer, and/or spacer (up to 2” / 50.8mm long) to fit these screws.
The intent of the rule is to allow teams to purchase their own commodity hardware without introducing additional functionality not found in standard VEX equipment. It is up to inspectors to determine whether the non-VEX hardware has introduced additional functionality or not.
The specific screws you have linked are commercially available, #8 size, shorter than 2" in length, and do not inherently introduce any additional functionality in terms of Robot design. Therefore, they are legal, provided that no other rules are violated by their use.
With that being said, we cannot predict every hypothetical way in which a fastener of a non-standard material type could be used in Robot design. As noted in R12, it will ultimately be at the inspector's discretion to determine whether or not a nylon screw has introduced additional functionality. In general, if the screw could be replaced with a standard VEX screw with no resulting functional changes to the mechanism in question, then they would be legal. However, if the mechanism takes advantage of a functional difference between the two screws (such as the additional flexibility of a nylon vs metal fastener), then they would likely be ruled illegal.