they cannot claim it as a derivative.

simple making a texture compatible with an existing character's UV template does not make that texture a derivative of the character.
nor does modeling clothing that fits a figure make it a derivative and the property of the figure creator. that entire line of reasoning is absurd. if that were the case, all daz products would be the property of Smith Micro right now.

aside from that, keep in mind that the reason Daz is in the position it is now is because of the add-on creators creating hundreds of thousands of add-on items and freebies for their products ever since Vicky 1. Vicky, Mike, etc would be absolutely nowhere if it was just Daz in-house supporting them for the last decade. so aside from the fact that it wouldnt fly in court for a heartbeat, if they started claiming all V4 clothing and textures were derivatives of Daz products they would alienate all add-on creators and theyd take a massive loss.