The Groovy style guide says, "... Groovy doesn’t allow you to remove parentheses. [...] for nested method calls or on the right-hand side of an assignment, you can’t omit them there."
Then, a code fragment is given, where just this works.
Further more, I could not find any examples, where this would not work, neither with comma separated parameter lists nor with infix computations nor closures.
So I guess it is just a documentation bug and right-hand side parens removal support has been added.