If you have ever cracked open a coconut you know that there is a lot of air inside along with the relatively hard coconut and a small amount of watery coconut milk. Alvarez and his assistants wanted to examine the possible effect of a jet of soft fluid-like matter. Brains are very soft. In fact they must be treated with chemicals just to harden them enough to permit examination if they are removed during an autopsy. Alvarez says that they wrapped the melons in Scotch filament tape to "mock up the tensile strength of the cranium". Undoubtedly a taped covered melon is not a perfect model of the human skull. But it is impressive the the first crude model of a skull that Alvarez made proved the theory correct. What really counts is the physics of the situation which you can read about in the link in #110.
Alvarez was not such a sleaze as to fake the test. In contrast to your previous assertion he says that 6 out of 7 tests resulted in backwards recoil of the melon. In the seventh test the melon "just rolled around". The reason I said "fell backwards" is that the film showed the melon on top of a step ladder, so that the direction of recoil would be the direction of fall.