Can you explain how that picture would be produced with 75 numbers per square? The generalization that seems obvious to me already fails in both cells of row 2, because 75 is odd:
You are completely right; their pattern holds iff (I believe, not proven!) the number they take 75 for is even. If you take any even number of rows (== amount of numbers per cell, in their construction), say 6, or 74, or 76, the pattern works.
Then their claim that this pattern holds isn't even true; as pointed out elsewhere, and is obvious when you write out the sequence items explicitly in terms of their coordinates, any gcd!=1 cell will be red. But a gcd=1 cell need not be black, for many n.
row 1, col 1: [1, 2, 3, …, 75], has primes
row 2, col 1: [76, 78, 80, …, 224], no primes
row 2, col 2: [77, 79, 81, …, 225], has primes