Every situation is different.
Jumping into the fray is fine, when it's known who the GG is and a positive outcome is likely. But few situations I've seen are so clear-cut along with having a likely outcome. In those such situations, it's simply a junkyard dog fight in which it's far smarter to call "God" (police) and let "him" (them) sort it out who's who.
A crime that occurs right in front of a person is one thing; a rape is clear-cut; a robbery in progress by a masked person at the corner store is clear-cut. None of which are really the point of the above quote.
Were we all omniscient and possessed of amazing H2H skills as well as willpower and a firearm, then correct assessment of all situations would be achievable and "helping" could result in the right thing being done. Life isn't always so straightforward.
(Note: None of this presumes to imply that all clear-cut situations are to be ignored and obvious GG's discarded to the winds of karma as if acknowledgment of this distinction applies to every situation. Consider that, before broad-stroking claims of "cowardice" to those attempting to do the right thing.)