Any time you pay someone an amount, you could be said to be "stealing" income from them by not paying them more. But the fact is that these MailChimp employees were not owed more than they were paid, and were not entitled to it under any widely accepted economic or legal theory in the United States, where they are employed. They agreed to work for a certain amount, which by all accounts was compliant with minimum wage and other laws, and nobody has alleged that they were paid less than that.

As far as being lied to goes, people would be better off if they replaced "never" with "not in the foreseeable future" any time they hear it. Who knows whether the founders genuinely believed their statements about never selling when they said them. Probably so, but if not, well, it's unfortunate. That being said, I doubt that anyone worked at MailChimp mainly because they believed it would always be privately held.