How does the allegation make any sense?
AI labs can hardly just throw random confidential data into the training and then hope it does not leak into the output of their model in an obvious way.
If that would be found it would destroy their main source of revenue, it could became a major national security or healthcare enforcement matter, and result in criminal investigations.
That's not at all how they do it. They wash the data. The end result is they can steal your IP insights without it being explicitly tied to your business. All of the decisions you made to build your product? Those become the standard suggestions in the next model for people building the same sorts of products. All of that error correcting you did, which in a normal business would be considered hard-earned IP (like in the case of Apple's lawsuit - the "what not to do" is just as valuable as the "what to do"), is now free correction for the next model to produce the perfect result in a one-shot. Now the AI company can commoditize your labor and your industry, or compete against you if they so wish.
Some of the smartest people on the planet all in the same room, data at their fingertips… they randomly add it to the training set?
Labs at least must study prompts in an airgapped fashion. From there, consider how they could generate synthetic data to train another model. After, require trusted staff to do multiple levels of independent granular reviews of all fruits of the highest-value stolen inputs. (Or for model training data only, data never has to leave the airgap.)
Definitely risky, anyway. Surely some AI user has sent data, in confidential mode, with a unique shape they expect to be able to recognize if a later model recreated a facsimile even with heavy substitutions… but labs could bring risk of getting caught (over next few years) down quite low with extraordinarily ultraparanoid strategy. (But hopefully everybody is just behaving!)