Earlier quoted context omitted.
I think the real markers should be that a person is not afraid of broadening their views. Usually, the advice for hiring for Erlang, as an example, is not to hire people that know Erlang, but focus on hiring good people (who are capable of learning beyond your average C-like) and then teach them Erlang. It's a small and very easily learned language, for sure, but learning it is certainly a bigger undertaking than lea…
I've been trying to learn Angular2 recently, maybe I'm not a good developer (but I faired ok with Erlang :D), but it really is such a struggle. Documentation is lacking, and what is available is often wrong or outdated. It doesn't help that half the tools and libraries are still in 'beta' and breaking changes are seen as a way of life. This post really sums up my experience, and I've done plenty of Javascript before…
The decisions they made going from Angular1 to Angular2 are a big reason I've never really bothered with Angular.
Just on a base level, if a system isn't properly documented, that does make a case for finding someone with indepth knowledge.
But a company using a system that is very poorly documented as their core technology is a big red flag for me.