Dog whistle racisim, I think, is when the racisim is there but only recognisable by certain people. But that could be argued that it's people attaching racisim where there is non.
Although dog whilstle politics is that actual term. I can only assume MN are using the term dog whistle and applying it to racisim.
Not very flattering to those who spot it. Isnt it calling them dogs?