I can see that Figure 2.F.1(a) satisfies the WARP (Definition 2.F.1) in MWG (page 30). However, as the choice $x(p',w')$ is only feasible under the price-income level $(p',w')$ and $x(p'',w'')$ is only feasible under $(p'',w'')$ in Figure 2.F.1(b), why does it still satisfy the WARP definition which requires that a choice (say, $x(p',w')$) should be feasible under two different price-income levels (say, $(p',w')$ and $(p'',w'')$)?
Intuitively, this just says that if the bundle you choose was possible under wealth $w$ and price $p$ but it is not $x(p, w)$, then it must be that you cannot obtain it when price is $p'$ and wealth is $w'$.
It simply does not say anything about case (b) in which $p' \cdot x(p'', w'') > w$ and $p'' \cdot x(p', w') > w''$. Since the first part of the if ... then statement in the definition is false, everything evaluates to true.