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.
He does acknowledge that Postel's Maxim might be essential to any widely deployed protocol that wants to be successful. He also acknowledges that his alternative is inaplicable to the early life of a protocol.
The main two flaws in tghe reasoning is that incompatibility or bugs are not intentional and that success is contingent on something 'just working'. For a thousand-feet views, you want errors, whatever their source, to propagate as little as possible and affect as little of a network as possible. Postel Maxim provides that effect. Being strict ensures that some process somewhere over which you have no control will affect your system.
Fortunately, it's being applied everywhere, notwistanding purists. Your house electrical input gets filtered and aim to provide a standard volatge. Your computer power supplies filters that and aims to provide a stable voltage and amps. Your electronics are surrounded by capacitors... wand it goes up the stack. It's just good engineering.