I believe they mean it can't effectively be sold if everyone has it. It loses value as a commodity if anyone can access it, but the value of the data is still in tact.
But facebook is not in the business of selling your data. It's in the business of selling your attention and it uses data to do so. There's nothing about this leak that changes Facebook's position in this market in this regard.
I can’t find any Facebook resource that says I can buy data, or any other reputable source that reports Facebook is selling details about people. Can you provide any?
This one is interesting and refutes the argument you are making
“When the company argues that it is not selling data, but rather selling targeted advertising, it’s luring you into a semantic trap, encouraging you to imagine that the only way of selling data is to send advertisers a file filled with user information. Congress may have fallen for this trap set up by Mr. Zuckerberg, but that doesn’t mean you have to. The fact that your data is not disclosed in an Excel spreadsheet but through a click on a targeted ad is irrelevant. Data still changes hands and goes to the advertiser.”
The opposing view is that the leaked data is only valid up to the time of the leak, and there is no guarantee it doesn't go stale, while Facebook has the fresh data.