Its fair to say so, though it is true that Nasu decisions are heavily made so that the reader views her as a bystander yet at the same time shows hints to the contrary. So its hard for people to picture her fighting, so the discussion would naturally cite her trauma from the abuse, not wrongly but its manipulated to a degree, so I can see where kohaku may be coming from.
For example, when Rin and Sakura fight when Rin tells her to go home in the Fate route, they make up in relatively quick fashion after Shirou leaves the room. Given the two know each other as magi, something happened to dispel their earlier fight.
And Rin openly lies in UBW when she and Shirou talk about the Matous. We also know Rin checked Sakura for command seals, now whether that was her upfrontly asking or not wasn't explained. So my whole spiel is that things are complicated, moreso than just Sakura being traumatized by Zouken. Hell, its actually a point that Sakura herself offers to help despite her condition multiple times and is shot down for it, while simultaneously paradoxically lambasted over a lack of courage.
It was mentioned that Kohaku had layers, but I think people forget that Sakura's temperament isn't a simple, "I'm scared of Grandpa so I won't fight". Zouken offered her to take slaves if she didn't want to kill, and she refused. There are things like that which show how being in thrall to an evil magus, but not being evil yourself is hard. Her options are limited to say the least. Not necessarily a lack of will or courage, but a lack of means and tenuous allies to boot.