The stock sale announcement was to let us know the stock was already sold
The 3.3M shares the MTZ founders announced were already sold to some institution. The announcement was simply a "for your information," and to satisfy the SEC rules. So there is no danger of the stock getting into the daily trading to move the stock price down. But some trading seem to be treating it as a future sale instead of a sale that has already been concluded.
I think my assertion is wrong now. It does appear even though the press release says the stock is "sold," it really was an indication that the stock "is being sold." it was either an unfortunate use or intentional misuse of the tense of the verb "to sell."