The probability of the `b` being chosen for A is 1 (No other choice) and for B it is 1/2. The probability of `b` being chosen for both (The only common token), is |{(b,b)}| / |{(a, a), {(a, b)}| = 1/2 which is jaccard's similarity |{b}|/|{a, b}|

I could have explained that a bit better I suppose.