I suspect that it might be simple to make something like JS in 10 days when you can effectively delegate the heavy lifting of making it fast etc. (see last slide) to many other people after it has become ubiquitous. :)
I wrote my own interpreters for my own languages, including stuff used in production. 10 days for designing and implementing something like JavaScript is pretty good.
Also, I think that the number of problems in JS normalized by the time it took to make the first version (and that time limit was due to a deadline that the creator did not choose) is very likely smaller than in many, many other languages, if not all the remotely popular ones.
