Please look at Boris Johnson’s children and tell me if they look mixed race (relevant, because they don’t, and won’t have received anything like the targeted racist bullying that I’m referring to).
Also, what Pinkie said 
And howabout raises an important point about the “context” of racist comments. 99% of the time, the context is “racist comments being made by racist people.”
There’s a huge difference between white Boris making a racist comment about, say, Muslim women, and David Lammy quoting a racist comment that is directed at people of his own race.
In fact it’s so different that it would take quite a lot of mental gymnastics to conflate the two.