Even without the collecting time, I still doubt about the probability X. When the "selfish miner" begin the next block, they are not at the same base point as the public network.
There is no base point. Every hash is unique with the same probability of success.
Let's say the nonce is from 0~100, and each node has to find one nonce to satisfy the requirement. The attacker has to start at 0, maybe it's just the one he needs, maybe not. The probability is 1%. While for the public network, they may start at 50(cause they try the nonce all along), and the probability is 2% now. It seems clearly that they have different base point.