I think that when someone calls 999 to report that they've been attacked, officers arriving at a chaotic violent scene under high levels of stress are likely to go with that as an initial working assumption. It's not right that they do that, but human psychology is what it is.
(I believe that's the case because statistical evidence shows that it's the case.)
I believe that, if the perpetrator and a number of his family and friends conspire to all tell the same lie to the officers as they arrive, get their story straight, hide the weapon, and claim the perpetrator was attacked and defended himself, it's not inconceivable that the officers would be fooled initially until more evidence is gathered.
I can well imagine this happening whatever the colours of the victims, in fact I've witnessed it (with minor assault, not murder, of course). Police aren't perfectly rational robots, they are turning up to a scene with no idea what to expect, and make mistakes.
Neither of us knows what actually went on in the mind of those officers. The difference is that you've already decided the reason, and assert without evidence that it must therefore be true. Your argument appears to boil down to "I can't imagine it happening for a different reason, so it must be this reason." It's the thinking process of a toddler.
Lastly, I don't really need to "think about what would happen if he was black or brown", because (in reality, not in the imagination of people who have already made up their mind) there continue to be countless instances of such people being treated badly by police and other authorities. Nigel Farage and Tommy Robinson don't tend to mention them, though, so they probably don't take up much space in the minds of people who make statements like yours.