Most studies in general show no “significant” effect of protein supplementation.

Protein supplementation does seem to improve muscle mass gains from resistance training. But you need a meta-analysis or huge study (lots of subjects and training duration) to consistently show it. This study was much larger than average, but we still did not find it.

So it’s possible that it actually works. But of course this was a study with a special patient population. It’s possible they are less sensitive to the beneficial effect of protein.