The probability of completing a flush draw in 2 cards is 35% or 1.86-1. David was just being approximate in that section. The 35% figure is well known (e.g. see appendix in HEP or HEPFAP). Here is a link to
3 correct methods for doing this problem. Taking "half the 4-1 odds" for 1 card to get 2-1 is an approximation which happens to be quite close. Note that this is not the same as doubling the probability or doubling the outs, which would be a much worse approximation because it double counts the times we get a flush card on both the turn and the river.