The main point is correct, but maybe you should mention your demonstration is especially easy, but not necessarily the main reason (unless that’s what you think?). Also:
The fair-goers, having knowledge of oxen, had no bias in their guesses, thus the error was entirely due to random noise.
If you meant the crowd had no bias on average, that’s indeed the idea. But one can read your sentence as meaning that each individual had no bias, which would break the whole wisdom of crowd idea (because then Galton wouldn’t need a crowd: he could simply repeat the measurement process in one individual).
The main point is correct, but maybe you should mention your demonstration is especially easy, but not necessarily the main reason (unless that’s what you think?). Also:
If you meant the crowd had no bias on average, that’s indeed the idea. But one can read your sentence as meaning that each individual had no bias, which would break the whole wisdom of crowd idea (because then Galton wouldn’t need a crowd: he could simply repeat the measurement process in one individual).