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I hear this opinion frequently. It's this conjecture or is this actually ever said on a canon/legend source?

It doesn't make sense to me for a form derived from soresu to apparently have bad defense against saber wielding opponent's.

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.