The ruling says you can't discriminate who you sell to, but you can discriminate on what you sell.
The baker in question had sold generic wedding cakes to gay couples in the past and continues to do so. The baker in question refused to bake a 'gay wedding cake' with two men on it as decoration instead of the traditional man & woman.
The gay couples who had purchased generic wedding cakes from that specific baker had finished decorating the cake themselves with figures of their choice.