If the coin were unbiased, we could compute the exact probability of getting 10231 or more heads with 20000 flips as: "sum (20000 choose x)/2^20000 for x from

If the coin were unbiased, we could compute the exact probability of getting 10231 or more heads with 20000 flips as: "sum (20000 choose x)/2^20000 for x from 10231 to 20000", which Wolfram Alpha evaluates to 0.00056. The probability of getting a number of flips that differs from 10000 by at least 231 is twice that, so about 0.001. So, in fact, the probability of this happening by dumb luck is about 1/1000. That's pretty strong evidence.