So, they are claiming that only 429 people are killed by the police each year, when most other sources, including the BJS stats estimate three times that number (about 1200).
They are looking only at shootings. The sites that get numbers like 1,200 are looking at everything--including traffic accidents.
In addition, their analysis assumes that every single time an officer claims (usually w/o a shred of supporting evidence) that a suspect "assaulted" them with an "edged implement" or firearm, that the cops are justified in killing the suspect.
That's pretty much how it goes--someone comes after you with a blade or gun, you can shoot. Cop or civilian makes no difference--they operate under almost the same rules of self defense as we do. The big difference is that they can shoot even when they are the aggressor, we can't.
First, all sane people know that every time cops fire their weapon or use lethal force that they will claim that they were "assaulted" or threatened. Not only do they regularly embellish whether the suspect actually used the weapon, but often plant the weapon. And no, that ain't just Hollywood. Hollywood got the idea from the well known fact that cops do this. I personally have closely known two cops, and both told me the carrying an unmarked illegal gun is widespread among cops.
Note that the assaults exceed the dead.
Second, cops are not supposed to kill suspects, even when threatened. They are supposed to merely eliminate the threat, which most times would not require killing the suspect, unless the cop is incompetent. These "researchers" are comparing every time cops are in any way "assaulted" no matter the objective resulting harm to them, to only the times the cops actually kill the suspect. The proper apples-to-apples comparison is every time cops ever use any force that could have potentially resulted in death of the suspect, which includes at minimum, every single time a cop fires their gun, even if the suspect is only injured, not even hit, or gets away. The cop is reporting a supposed "assault" in all those cases, so they all need to be counted as the cops attempts use lethal force. Sadly, since there is no such thing as real police oversight in the US, there are no reliable stats on that, but the number is easily 20-30 times higher than the number of actual deaths (1200), resulting in 24,000 to 36,000 incidents where cops attempt to use force that could kill the suspect. That is 7-11 times the number of incidents where cops claim they are assaulted with with a lethal weapon.
Eliminate the threat + competent shot generally means the guy is dead. Shooting to wound is purely Hollywood.