In my experience, this claim is made by (many times really good, experienced, intelligent) coders who have a lot of previous experience in static typing and aren't comfortable -- either because of a lack of time and sheer LOC, or because of some mental block they've doubled-down on ("where's my 'extract method' menu option???") with the techniques and peculiar challenges of dynamic type refactorings.
I work on a very large application/system of applications in a dynamic environment, and I've never encountered refactoring or maintenance issues that I thought would have been any easier in my previous life, 10 yrs in static typing.
The other thing to keep in mind is that -- at least when comparing something like, for instance Rails to Java, there will be a lot less code, and this is a significant contribution to code management. That factor, plus just experience and acclimation to the techniques for dynamic-type editing/replacing/finding/greping and so on, plus solid TDD practices -- maintenance has never been an issue for me, and I've had a decent amount of time in large codebases in both dynamic and static environments.