It comes down to specific legal language of the various licenses, not an analysis of what Arm did in the past. The claim is credible enough that the court didn't dismiss it out of hand. Armchair analysis without the actual licenses and some legal knowledge aren't all that helpful.

The lawsuit states

"Nuvia's licensing fees and royalty rates reflected the anticipated scope and nature of Nuvia's use of the Arm architecture. The licenses safeguarded Arm's rights and expectations by prohibiting assignment without Arm's consent, regardless of whether a contemplated assignee had its own Arm licenses."

So Arm's case seems to hinge on whether or not the license language covering the prevention of assignment will be held up in court as applicable to this situation. That's the kind of legal dispute which will be very technical and not easy to speculate about... definitions of individual words and intent and all of that.