how to understand the MIMO zero forcing as a bank of decorrelators?
It seems like you understand your question already but are unsure it is correct? Let the decorrelator be $\mathbf{D}=(\mathbf{H}^T\mathbf{H})^{-1}\mathbf{H}^T$, then the symbols post-decorrelator are $\mathbf{z}=\mathbf{D}\mathbf{y}=\mathbf{D}(\mathbf{Hx}+\mathbf{n})$. The first stream, $z_1$, is computed by taking the first row of $\mathbf{D}$, call the row vector $\mathbf{d}_1$ (first row of $\mathbf{D}$), and performing the operation: $z_1=\mathbf{d}_1(\mathbf{Hx}+\mathbf{n})$. You get $z_1=x_1 + \text{filtered noise}$ because $\mathbf{d}_1\mathbf{H}=[1, 0, ..., 0]$.