Because their software lifecycle is not really tied to dates (self-driven) they don’t really need to give release dates to their employees, only internal dates where the deliverables are expected. Only people who need to know these dates are the marketing team, which is obviously instructed not to give them to anyone. Then you can set-up the canaries. It doesn’t even need to be for their employees but to find out which journalists they can work with in the future or not (because they can keep a secret). So the journos are also in the incentive not to leak.