First, it is always recommended to use pasteurized egg whites. Having said that, not all of us have access to these pasteurized egg white cartons. Therefore, this traditional method of making Swiss meringue buttercream is better.  Sugar starts to dissolve at 140 to 160 C, and the temperature at which the salmonella bacteria is killed is also 160 C. So, the general conclusion is if the sugar has melted in the eggs the mixture has reached 160 C. And so the bacteria if any have been killed.