It’s not a placebo. I’m not sure exactly which Chocrane review the GP is talking about but if it’s the one I’m thinking of, the headline is that stretching doesn’t help muscle soreness [1] (delayed onset muscle soreness specifically).

You should absolutely keep stretching to help with your injury, and there is decades of “sport science” to back that up.

[1] https://www.cochrane.org/evidence/CD004577_stretching-preven... - there’s another one about joint deformities which is far more interesting, but less relevant to this discussion