I think the productivity gains of dynamic typed languages were real, and based on two things: dynamic typing (can) provide certain safety properties trivially, and dynamic typing neatly kills off the utterly inadequate type systems found in mainstream languages when they were launched (the 90s, mostly).

You'll notice the type systems being bolted onto dynamic languages or found in serious attempts at new languages are radically different than the type systems being rejected by the likes of javascript, python, ruby and perl.