No, this is not the only example of a language that was developed de novo. When deaf people are born in an environment where sign language is used, they acquire it just like hearing children acquire spoken language. However, between 90 and 95% of deaf people are born to hearing parents. Parents are met with a variety of early intervention options, and sometimes choose not to teach their children sign language opting instead for spoken language based interventions (hearing aids, speech therapy, cochlear implants..etc). Sometimes the spoken language interventions fail, and the children (no longer babies) have not been exposed to language at all. One of the most remarkable discoveries is that these children will invent idiosyncratic signing systems, and these systems are actually much more systematic and complex than the gesture systems that even their mothers use. Essentially, these kids are inventing a linguistic system (albeit simplistic) on their own. These home sign systems can sometimes become languages through two mechanisms. In the Nicaraguan case, a school was established and they came together and over the past few decades their idiosyncratic systems became conventionalized and much more complex. New students come to the school and learn the language from their peers. This is an example of a community sign language. The other way this can happen is in families that are genetically deaf. If these families are in a relatively isolated location, such as Al-Sayyid Bedouin Sign Language users, they can develop a language over the generations. Children learning village sign language learn it from their parents from birth.
For this reason, I think despite the fact that Nicaraguan Sign Language originated with home signers, it is fair to say that it developed from nothing.
Nicaraguan Sign Language has been well studied because researchers were there from the inception, and have been able to look at its genesis. There are a handful of village sign languages that have been documented. Al-Sayyid Bedouin Sign Language is one of the better documented village sign languages. I am working on a new one in a village in Turkey called Central Taurus Sign Language that we discovered last year.
Sandler, W., Meir, I., Padden, C., & Aronoff, M. (2005). The emergence of grammar: Systematic structure in a new language. Proceedings of the National Academy of Sciences of the United States of America, 102(7), 2661-2665.
Senghas, A. (1995). Children's contribution to the birth of Nicaraguan Sign Language (Doctoral dissertation, Massachusetts Institute of Technology).
Mylander, C., & Goldin-Meadow, S. (1991). Home sign systems in deaf children: The development of morphology without a conventional language model. Theoretical issues in sign language research, 2, 41-63.
Carrigan, E. M., & Coppola, M. (2012). Mothers Do Not Drive Structure in Adult Homesign Systems: Evidence from Comprehension. In N. Miyake, D. Peebles, & RP Cooper,(Eds.), Proceedings of the 34th Annual Conference of the Cognitive Science Society (pp. 1398-1403).