"Dog whistle" is a whistle with a high pitch that only dogs can hear. It also means something that can be said, which isn't overtly racist, but which racists will recognize as reflecting a racist attitude.

In this case, the comment is accusing you of summoning a dog with your dog whistle, while meaning that your comment is subtly racist.

For what it's worth, I don't really agree. The bombers are largely second or third generation immigrants, and I read you as implying they weren't ethnic Swedes - which is true. I don't see how the truth can be racist.