So, is wiki.c2.com something that people keep writing on or is it really readonly nowadays? Is there a way to become a member? I just mistakenly signed up to hypothes.is from the right side-bar thinking it was c2.com.

Seems like it hasn't been updated in a while. Each page has an "last updated" timestamp. The homepage says: "Last edit December 19, 2014". There's also a Github project but hasn't been updated in 2 years: https://github.com/WardCunningham/remodeling