Fortunately, Postel's law is not applied everywhere. Electrical deviations are formally specified so engineers design smaller error bounds, resulting in lower cost of the end-product (or lower weight and size). Your computer power supply will still produce magic smoke when connected to 330V or when subjected to strong fluctuations. The 'liberal' part is actually the 'strict' part: your electricity provider is obliged to provide you with an electric potential within certain boundaries.
The reasoning of the author is simple: we want a POC of an idea ASAP (lacking formal specifications of anomalies and error bounds) and when successful, error bounds and boundary conditions including specifications thereof should be communicated and implemented. That seems like a cogent and professional point to make, given the complexity of our systems.