I am curious for why people say this as in my experience when writing a function in a dynamic language that takes a variable as a parameter:

The function will generally only work on a subset of types for given variable.

If I don't check the type of the variable in the function, the function will not behave as you might expect, e.g. silently fail or crash.

If I do check for every possible type for a given variable:

I may not have a good way of handling certain types being passed in, I may be forced to either log something out, create a run time crash or even have the function silently fail. All 3 are bad run time behaviours.

If I am checking for every type in the functions, then using static typing would cause the failure at compile time so the bugs could never exist, but also being significantly less verbose than the dynamic language equivalent.