Good find. Key paragraph:

When measured objectively over the 42-day evaluation period, limb function improved in 12.1% and worsened in 8.6%, but did not change in most dogs. By contrast, caregivers (both owners and vets) reported improvements in lameness from the start, with the reported improvements increasing with time. The caregiver placebo effect appeared to be around 57% for owners and 40–45% for vets and was statistically significant at all assessment time points.

Objective measurements are one way to detect this effect. Another would be a true double-blind trial where neither owner nor vet knows which medicine was given.