My recent experience says this is not true. Python is so widely used as a backend that it is in no kind of secret weapon - at least not in the way Lisp was a secret weapon for Paul Graham with Viaweb. Every startup I've dealt with recently is building their backend in Python, Ruby or Node. I think the real story here is that compiled languages are on a major downswing in use for startups that build web apps. I haven't run across a startup building their backend with .NET or Java in a couple of years, except for one payment application that built the business logic with C# and MS SQL Server (they still built their web API with RoR).
Recently I tried to go back to compiled languages for a new web project. I really wanted to try something fun in Haskell that wasn't just data processing (what I normally use it for). I picked up Snap, Yesod and a few other libraries only to find that it was just too much of a pain.
Dynamic languages remove so many little annoyances with web development that it's hard to justify going back to a compiled one when you could just cheaply spin up an extra VM or two to deal with the performance differential between dynamic and compiled languages.
Maybe I'll try Go next...