In the previous day’s comic, both Clarence and Sam used the correct terminology, “to sign.” I dont think any audience would not understand “I sign American Sign Language,” so I dont understand your point
“Sign language” is like saying “spoken language” – it is a medium, not a specific language. There are over 300 sign languages in the world and THREE that originated in the USA and are still signed today – ASL (American Sign Language), PSL (Plains Indian Sign Language), and ISL (Inuit Sign Language). The correct way to say that you know a sign language would be like this: “I sign ASL” not “I speak sign language”
It has to do with thickness of individual strands – thick filaments = hair, thin filaments = fur