This answer presents a derivation of the value of ionic product of water at $25^{\circ}\text{C}$.
The relation $K_\text{eq} = \operatorname{e}^{-\frac{\Delta_\text{r}G^{\circ}}{RT}}$ is used for the derivation.
But why is the equilibrium constant $\mathrm{K_w=[H^+][OH^-]}$ and not $\mathrm{K_d=\frac{[H^+][OH^-]}{[H_2O]}}$ or $\mathrm{K_i=\frac{[H_3O^+][OH^-]}{[H_2O]^2}}$?
After all, the reaction $\ce{H2O <=> H+ + OH-}$ is used for calculating $\Delta_\text{r}G^{\circ}$, so intuitively it appears that $\mathrm{K_d}$ should be used.
How can using any of the equations be equivalent when we have specified which reaction we are using while calculating $\Delta_\text{r}G^{\circ}$? What am I missing?