Maybe. But the data is there, imagine financial troubles, someone buys in and uses the data for whatever they want. Much like 23andme. If you want something to stay a secret, you don't send it to that LLM, or you use a zero-retention contract.
If your threat model is "vendor willing to ignore contracts and laws to steal your data" I can't see how a zero-retention contract helps.
If your threat model is "vendor willing to ignore contracts and laws to steal your data" I can't see how a zero-retention contract helps.