No genetic bottleneck proves no global flood

Well, you look at the size of the population and the amount of genetic diversity within it. In equilibrium (i.e. if the population has been about the same size for a long period of time) the diversity will be proportional to the population size --- there'll be a certain quantity of diversity such that the production of new variants by mutation is just balanced by the elimination of variation by genetic drift.If there's markedly less diversity than that, this indicates a recent bottleneck.