It is true! They announced this on my flights too. Does not apply to business class and since I wasn't in economy, I cannot comment if people paid attention to this rule.

On my flights, there was also no boarding/de-boarding by seat row. The crew made frequent announcements but nobody cared. The crew also didn't close the curtain between Business and Eco so once we arrived at our final parking position, the Eco was already in the Business Class