MOSFET drain current equation vs on resistance
The \$R_{ds}(on)\$ spec doesn't apply to saturation mode.

It applies in the linear or triode operating mode, where the current through the channel depends strongly on both \$V_{gs}\$ and \$V_{ds}\$. That does mean that \$R_{ds}(on)\$ is not really a fixed number but depends pretty strongly on \$V_{gs}\$, and the \$V_{gs}\$ where the specification is measured should be stated clearly in the datasheet.

For example, in the Vishay IRF530 datasheet: