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Suppose, I have a hardware random number generator that tends to set some bits to 1 and some other bits to 0. If the attacker does not know which bits are biased and what their biases are, they have no information that will help them.
You can say that flipping a coin is not truly random, but it is effectively random to the attacker unless they know the exact conditions that were present when the coin was flipped.
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Agree with you.
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And I beleive that every human flipping a coin makes it in some pseudorandom way because of his habits, physical conditions, etc. And in fact, all these different singularities would add the additional entropy for the key generation process (like one man will make the flips with 51%/49% odds, another with 48%/52% odds, etc
In most cases the attacker will not know the biases. However in case he knows some bits are 52% instead of 50%, this information will not actually help in practice, because the total possible combinations still will tend to a very big number close to 2^256