Numerical Stability and Initialization

http://d2l.ai/chapter_multilayer-perceptrons/numerical-stability-and-init.html

\begin{aligned} {E}[h_i^2] & = \sum_{j=1}^{n_\mathrm{in}} \mathbf{E}[W^2_{ij} x^2_j] \ \end{aligned}

I’ve found the second equation of 4.8.4 hard to understand. Let’s say h_i = w1 * x1 + w2 * x2, then E[h_i^2] = E(w1^2 * x1^2 + w2^2 * x2^2 + 2w1 * x1 * w2 * x2]. The above equation simply abandons 2w1 * x1 * w2 * x2?