Don't believe such a thing's ever stated. Lightsabers: A Guide to the Weapons of the Force claims that it uses parries in order to force their opponent to drop their guard. It's clearly designed for dueling in part, even if it's mostly just a more offensive version of Soresu.
I don't know if the form's derived from Soresu, though. I do know that it was made in response to Form III's offensive limitations.
Im not entirely sure if the phrase "weak against" has ever been used, but it's strongly implied.
"Form V’s Shien variation exploits the ability of the lightsaber to block a blaster bolt, and turns this defensive move into an offensive attack by deflecting the bolt deliberately toward an opponent. Form V’s Djem So variation is devoted specifically to lightsaber dueling."
- Jedi vs Sith: An Essential Guide to the Force
The focus on Shien's ability to block blaster bolts, juxtaposed with Djem So being a duelling form, implies that what you're asking is indeed true. While you can call the form "derived" from Soresu due to its placement in history, they're distinctively different forms, and Jedi vs Sith elaborates that they were invented to cover the weaknesses of Soresu and allow Jedi to face different groups of people.