The "dickmove" comment may or may not be true, however, I'm not sure what details the backup officers had upon arriving at the scene. Did they know how many shooters there were? Did they know whether or not Stacy had joined the active shooter in firing against the first officer? They arrived and probably saw a man with a gun, right after one was shooting at an officer. In the interest of personal/public safety (and probably department protocol), I can't say I'd have done anything differently. He doesn't seem to think it's a big deal, and was released fairly quickly too.
I think you've got the wrong view, here. The question isn't, "Was there a reason not to detain this guy?" The question is, "What clear, identifiable circumstances existed that so overwhelmed the presumption of innocence that the officer reasonably felt obligated to detain Stacy?" See where you've got this backwards? There's no need to demonstrate why this guy shouldn't have been detained. The burden of proof is on anyone, particularly the dickmaestro involved, to demonstrate why this guy
should have been detained. Because so many people have their perspectives ass-backwards, we see this kind of thing, shrug, and don't even consider it. This perpetuates and strengthens the extravagance of police response in all circumstances, and emboldens the bad cops, who are then covered by all the other cops who would otherwise be "good", many of whom then decide to get in on the action.
N.B.: I'm aware that the presumption of innocence is a judicial standard, not explicitly an enforcement standard. The very reason we have the explicit judicial standard is because we are supposed to hold that standard, and more, in every aspect of our society. The framers simply felt it so threatened in the judicial arena that they took the extra step of explicit stating it in that context. This is like the Bill of Rights not being the only rights we have, merely the ones that were most urgently in need of explicit enumeration.