Wijnaldum and Frenkie de Jong pointed at their skin to the crowd after scoring the first goal of the game.
It is a byproduct of an incident that occurred involving Excelsior's Ahmad Mendes Moreira, who suffered racist abuse from Den Bosch fans in a Dutch second division game.
"Gini just said something about it in the dressing room," Koeman told reporters.
"He said he feels so happy with this Dutch team, where this problem [of racism] does not occur.
"It is one team and it's nice to see."