It adds up, to be honest. She seemed to be a lot more secretive than we initially thought. In an earlier episode, when Greg said, "I barely know you," she said, "That's a good thing."
She sliced through Bismuth's physical form due to the Breaking Point and what it was made for. This leads me to think that the weapon only reminded her of her betrayal and what she had done to Pink Diamond. Maybe she's so anti-shatter because of what she did.
This especially doesn't help Rose's case.
There's just a lot of stuff that supports what that Ruby said.