'blood in his mouth' could be due to anything though - bit lip or something from falling over or scuffle. Again, on the video, you see the stabber tell the police officer HE is injured - part of their role is to try and work out what is true - if HN wasn't so severely injured they would have asked more questions and eventually established what had actually happened/that he was injured in sufficient time to do something. If he hadn't been seriously injured then talking politely to everyone, keeping the situation de-escalated, taking initial accounts rather than poking holes in inconsistencies, rather than jumping in and handcuffing everyone, is exactly how they should handle that sort of situation.
Or, if he was more obviously injured - i.e. completely unconscious, wearing lighter clothing so it was more obvious he was bleeding, had been stabbed somewhere else - they would have taken more reactive action straightaway.
Unfortunately it seems like just a perfect storm of events whereby in retrospect the way they handled it looks awful - but they didn't have that benefit of hindsight.
You also don't know how much of what the person told the police call handler was passed on to the dispatcher and then to the attending officers and how accurately.
the other thing to take into account is that it is very unlikely an ambulance would have attended a) at all until they'd definitely established he'd been stabbed and b) fast enough to make any difference.
Again - I'm not saying there's no room for criticism. But I'd be highly surprised if the individual officers had any serious misconduct actions brought against them. It's incompetence rather than malice, and, if there is an element of reverse racism again criticism of this should be aimed far more at the senior staff enforcing these policies rather than the staff on the ground struggling to often combine contradictory philosophies while severely understaffed.