I used to have that opinion as well, but I am slowly retracting it. Very strong typing and great abstraction does make development easier. E.g., I have been writing parsers lately for an application-specific macro language. Writing such parsers using a good Haskell parser combinator library such as parsec is many times more productive than parser generators for most imperative languages.

Another advantage is that strong languages allow you to refactor code easily. If you change some types, the compiler will find type mismatches, allowing you to fix code rapidly. I can imagine that startups often refactor and substantially modify their codebase.

In the end you do not only want to develop an application quickly, but also correctly. If a strong language can be used to eliminate most errors and improve productivity, I see it as an asset to a startup.