Populism is a precise way of appealing to basic emotions of a population by creating an "us vs them" narrative. You find an enemy, tell the people that the enemy is causing all their troubles and that you're the only cure because you're the only one on the people's side.
With a bit of perspective it's easy to see that this broadly describes socialism - which is the parents point. Socialism and social democracy is not commonly derided as 'populist'.